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First, as to acrylics. The pros of acrylics are that they dry quickly (which can also be their con). The positive side of a quick-drying medium is that you can lay down a series of glazes that dry quickly, allowing you to reinforce the colour that you've laid down, or you can glaze another colour on top of the previous layer and get yet another colour. As well, you can lay down a more impasto passage (which, of course will dry quickly) and then glaze over it. To extend your open time with acrylics, you can add Retarder and various acrylic mediums, but even when these are added, acrylics dry much faster than oils, so you do have a sense that you're racing to get your colours on the canvas before they dry up on your palette.
The most interesting feature of acrylics is that, compared to oils, it's faster and easier to create a variety of effects in one single painting. For example, some areas can be left with only the ground or imprimatura showing; other areas may feature single or multiple glazes or may be built up with more impasto work, while still more passages may be rendered with glazes laid over the top of impasto areas. In addition (since acrylics dry so quickly) you are less likely to mix colours on the canvas using the wet-in-wet technique, and instead use scumbling and dry-brushing to create colours that mix together on the canvas visually when you stand back and view your work from a distance. All of these different techniques, when combined together, can create an interesting effect, whereby some passages appear quite transparent while others appear much more opaque. The overall visual impact is a push-and-pull of transparency juxtaposed against opacity. So, within one single painting, you have a variety of applications of paint: exposed ground; glazes; impasto work and dry-brushing. And, of course, acrylics clean up with water, so one doesn't have to be subjected to the odour of solvents when cleaning brushes.
Now, the downside of acrylics. Acrylic polymer emulsion just can't carry the same pigment load as oil paints. The result is that even the most opaque acrylic pigments are somewhat translucent and don't offer anywhere near the coverage or opacity of most oil-paint pigments. Since oil pigments are suspended in linseed oil, colours from the tube are very concentrated, much like tube watercolours. You can squeeze out a very small amount of oil paint -- perhaps the size of a finger nail -- and you can cover a huge area of your canvas. Acrylics, on the other hand, just don't have that level of concentration, so you need about three to four times the amount of acrylic paint to cover the same area as you would with oil paint. Thus, with an oil painting, you can easily cover a large area in one single go, reducing the number of layers of paint you need to apply to achieve opacity. With acrylics (because of their translucency) you need to work in layers in order to achieve opacity. However -- and this is a big however -- acrylics have a limit in terms of the number of layers of paint you can apply. One layer will likely look somewhat translucent; two layers of paint over the same passage will achieve greater opacity, and three layers will certainly get you where you want to be. More layers than that, however, will degrade the look of the painting. If too much paint is applied, the surface takes on a strange shiny, gummy appearance, and well . . . the painting is now ruined.
Another downside of acrylics (that stems from their translucent property) is that whatever colour you lay in initially as your ground will affect any further colours you lay in on top. For instance, if you lay in an imprimatura of red, and you try to lay in a passage of green over the top -- even if it's a thick layer of paint, composed of opaque pigments -- the red (since it is complementary to green) will reduce the intensity of the green, greying it down and making it look muddy. Since oils are much more opaque, this isn't so much a problem. You can lay down whatever colour you want as your ground and then lay down the same or a different colour over the top, with the result being that the top colour is relatively unaffected by the colour underneath it.
Another feature of acrylics is that they dry at least one shade darker than when they are applied wet to the canvas. This is because the acrylic polymer emulsion in which pigments are suspended makes colours appear lighter than they are when dry. Oils, on the other hand, remain unchanged in value throughout their use. The colour that's in the tube, on the palette and on the canvas both when wet and dry is the same. Acrylics, however, darken noticeably when dry, so you need to make allowances for this value shift by mixing paints on the palette and applying them to the canvas slightly lighter than what the painting calls for.
As an aside, an interesting characteristic of acrylics is that pigments tend to be either quite transparent or quite opaque. Dioxazine Purple, Phthalo Blue, Phthalo Green and any of the Quinicridone, Hansa, Naphthol or Pyrrrole pigments are transparent (or at the very least, translucent), while Titanium White and the Cadmium pigments are opaque, but as mentioned, are still more translucent than their oil-painting counterparts.
One final note on acrylics is that, unlike oils, they're ready for varnishing relatively quickly. Once an acrylic painting is completed, it takes about two to three weeks for the paint to completely dry, at which time it can be varnished. Oil paint, on the other hand requires a lengthy period of drying before it can be varnished: at least six to nine months. The fast drying time of acrylics is due to the fact that, like watercolours, acrylics dry through the evaporation of the viscous content of the paint, namely water, acrylic polymer emulsion, and whatever mediums have been mixed in. By contrast, oils dry through the process of oxidation, which takes several months to complete. This is the main reason why acrylics dry almost instantaneously, and are ready for varnishing within a matter or days or weeks, while oils take the better part of a year until they are ready for varnishing.
Now, as to oil paint. What's nice about oils is, as mentioned, their coverage and their slow drying time. Regarding the coverage they offer, for opaque passages, you need to apply just one layer of paint, and you're done. Accordingly, you need far less paint to complete a painting than you would with acrylics. Also, due to their slow drying time, rather than dry-brushing or scumbling to create multi-coloured passages, you can lay in one colour, then using the wet-in-wet technique, paint another colour directly into the first colour on the surface of the canvas. The acrylic method of dry-brushing and the oil (and watercolour) technique of wet-in-wet achieve pretty much the same visual effect when you stand back and view your work from a distance; but with acrylics, you're always racing against time to get the paint down, since it dries so quickly. With oils, the open time is hours if not days, so you can take your time and not have to feel rushed.
But which medium is more forgiving, and provides the artist with opportunities to alter areas that aren't working out? And, which medium allows for greater improvisation? I would say that both mediums require that you have a plan of attack before you lay down even your first brush stroke. You need to have a pretty good idea as to what your figure-and-ground relationships will be: that is, how your composition will incorporate the light-on-dark-on-light formula so that you don't get lost edges. Oil is perhaps a little more forgiving, since you can easily wipe away an area that's not working out, and go back in with a different colour scheme, technique or effect. Conversely, like watercolours, once acrylic paint is on the canvas, that's it: the paint is dry, and you're pretty much committed to the colours and the technique you've chosen to render that particular passage of your work.
With regard to incorporating different techniques, in the world of oil paint, you're more likely to paint relatively opaquely and are less likely to use a variety of techniques, such as glazing and dry-brushing, since you have to wait for a couple of days between applications of paint. As discussed, acrylics lend themselves to glazing and other techniques that are made possible by a short drying time. The overall effect of acrylics is a hybrid of watercolour effects and elements of opacity that are similar to the look of oil paintings. By contrast, the overall effect of oil paintings -- because they most often lack the juxtapositions of glazed and impasto areas that are so characteristic of acrylic paintings -- tends to be that works look opaque.
One downside of oils, of course, is the clean-up. Whether you're using Mineral Spirits or Turpentine, you'll be using a petroleum product to clean your brushes. As to mediums, a low-odour medium for oil paints is Walnut Oil, produced by M. Graham & Co. The old-fashioned formula of 1/3 Stand Oil (or Sun-thickened Linseed Oil), 1/3 Turpentine and 1/3 Dammar Varnish dries with too much sheen for my liking, while Walnut Oil is more matte when dry, although it dries slightly slower than Stand Oil. Other mediums for oils are, of course available. For example, Venice Turpentine creates beautiful glassy glazes, but due to the slow drying time, if you're working with glazes in oils, you're better off working on multiple paintings simultaneously so that you can go back and forth between paintings as you wait for glazes to dry before you can go over them with more glazes or with thicker layers of paint.
One thing that I've noticed about oil painters who try to paint with acrylics is that they get frustrated very quickly. Just as pastels and watercolours are very different mediums, so are acrylics and oils. You can't take your skills and techniques as an oil painter and apply them directly to acrylics for several reasons: as mentioned, acrylics dry quickly, so you have to work quickly, without a lot of time to stop and reassess how things are turning out. As well, with the exception of simple glazed passages, acrylics require multiple layers to build up the opacity necessary to create impasto passages. This race-against-time approach to painting is virtually alien to oil painters, since oils allow you to work slowly and permit you the luxury of taking breaks so that you can pause and evaluate whether or not certain passages -- or the whole painting -- is coming together. Also, artists who have painted only in oils are used to the convenience of most oil pigments being opaque, providing them with the opportunity to block in large areas of a canvas with only a small amount of paint -- which brings me to the issue of affordability. Oil paints -- particularly high-end brands such as Old Holland, Natural Pigments, Michael Harding, Vasari, Williamsburg and Blue Ridge -- may seem pricey, but the paint goes such a long way. In fact, I still have tubes of Winsor & Newton oil paint from 30 years ago -- and they still haven't dried up. By contrast, while good-quality acrylic paints (such as Golden, Tri-Art, Winsor & Newton, Lascaux and Liquitex) are the best that you can buy, you end up using so much more paint due to the lack of coverage that acrylic paint offers. So, if you're looking for an affordable medium, oil paints are more economical in the long run, since you can produce a lot more paintings with the same number of tubes of colour as acrylics.
Perhaps one deciding factor with regard to choosing oils over acrylics is that acrylics are virtually useless for doing realistic, representational portraits. If you're doing still lifes or paintings of wildlife, house pets or landscapes, acrylics will do the job. But for doing head-and-shoulders portraits, oils (as well as watercolours and pastels) are the way to go. The reason is that when doing portraits in oils, you're best off using Lead White rather than Titanium White. The reason is that Lead White lightens colours without greatly reducing their intensity. Conversely, Titanium White both lightens colours and greatly reduces their intensity, and has the tendency to make colours mixed in with Titanium White look grey and chalky. Thus, using Titanium White in a portrait doesn't work because skin tones very easily take on a ghostly pallor when you try to lighten them by bringing white into the mix. Lead White, on the other hand, works well, since you can lighten flesh tones while retaining most of the vibrancy of colours mixed in with it. So, the formula tends to be: acrylics for still lifes, landscapes, cityscapes and animals, and oils for, well . . . pretty much everything, including portraits.
My advice is that if you're impatient, and you want to complete a painting quickly -- or if you like to work on-location "en plein air" -- oils (or watercolours) are the way to go. Oils dry slowly, so you have to wait about nine months before you can varnish them, but the painting phase is accomplished briskly and easily. Acrylics, on the other hand, require time. Yes, layers of paint dry quickly -- within minutes -- but you have to build the painting up with so many layers that you can quickly get bored. As discussed, glazed passages can be completed easily, since each layer of paint dries quickly, and transparent colours are highly concentrated; but rendering more opaque passages tends to be tedious, since multiple layers of paint are required to achieve the level of opacity needed for the visual effect you're trying to create.
So, acrylics versus oils? I bounce back and forth between both, but my favoured medium remains oils.