I've read several times that a 50mm lens will make features like noses noticeably stand out compared to an 85mm lens. However, I'm never sure if the post/article is describing results on a full frame or crop sensor. I know that the longer the focal length, the more the compression to features....because the crop sensor gives a 50mm a longer focal length, how noticeable is the facial distortion on say, a Canon 550D? I'm considering a prime for portrait work and I'm 'at the crossroads' like all of you who came before me ;)
EDIT I will generally be using the lens for wedding portraits on the beach (and they generally want to get married around sunset, so formal portraits come as I'm losing light---hence wanting a tack sharp f1.8 or below), so it would be handy to have a lens that can take more than just a head/shoulder shot.
You are correct that a 50mm lens on a cropped sensor camera will be more suited to close-up photography than on a full frame sensor. The smaller sensor size means that you don't need to get as close to the subject to fill the frame, meaning there is not the same distortion of the nose in particular.
Having said that, the answer really depends on your personal style; if you can get your hands one one of each types of lens (e.g. rented, or in a camera shop) and try them out on a willing subject (you could take a friend with you to the shop :D ) you'd get a better idea of which lens works best with your style.
For headshots, 85mm will perform much better than 50mm. If you shoot family portraits, then 50mm might be a better option.
Apart from the focal length, you might want to look into the out of focus control or number of diaphragm blades as they determine the quality of the bokeh.
I've read several times that a 50mm lens will make features like noses noticeably stand out compared to an 85mm lens
What you read was likely referring to full frame or equivalent focal lengths of 50 and 85mm.
In any case a 50mm lens on an APS-C sensor will allow you to take well framed portraits at a distance which doesn't cause any undesirable perspective effects on noses etc. I have shot a great many three quarters shots with a 50mm lens on APS-C.
Whether you chose 50 or 85 is a matter of preference, how you prefer to work and how much space you have. Personally I like to go longer (as it tends to give you more subject separation from the background) but it can be a pain when you don't have room!
The focal length doesn't affect distortion or compression as you're describing. Only distance to subject can change that.
What focal length and sensor size affect is the magnification of the subject from a given distance. If you have a lens at least as wide as 50mm, you can determine for yourself what you want. What do you want your working distance to be? Take sample photos from that, as well as other distances. Find the distance you find most flattering. Determine the crop that you need to apply to make the subject as large in the frame as you want. From that, you'll be able to determine the best focal length for your working distance and your tastes.
I think Eric's answer is what I have been trying to justify to myself. I'm going to try to explain it, and I hope I get it right (I know people will correct me if I'm wrong!).
That is, it is the distance that determines the compression/disortion, not the focal length. However to get the same framing on a crop-sensor vs. a full-frame sensor, you'd need to change the distance if using the same lens on each camera.
In other words, a 57mm focal length on a 1.5x crop sensor, should provide the same perspective and framing as an 85mm focal length on a full-frame sensor if shot from the same distance.
Now, it seems that 85mm-105mm is most recommended for portraits (full-frame), so a 50mm on a crop-sensor might be too wide (you'd have to get in tighter and introduce more distortion). I think this is why you'll see people using a 70mm (like a 70-200mm) for portaits on a crop-sensor (yielding an effective 105mm on full-frame).
I'd suggest you take one of your existing zoom lenses (or borrow one) and set it on 50mm, shoot for a while, then set it on 85mm and shoot for awhile. Think about how well the focal length suits your shooting style. Then... look at the images and see whether you find the 85mm shots more appealing than the 50mm ones.
I shoot with an 85mm f1.2 and a couple of things: You almost never need that fast a lens and the cost may not be worth the number of shots made at that aperture. The depth of field at 1.2 is not sufficient to shoot pictures of people unless you are looking for a specific selective focus effect. One thing (and I'm using a full-frame DSLR) about shooting with the 85mm is that I find myself backing into things sometimes to get enough space to work. That's why I recommend you try the focal length in your intended shooting environment. If I'm feeling cramped with an 85mm, I imagine you would feel more cramped using a crop sensor in the same circumstances.
A 50mm on a cropped sensor behaves exactly like an 85mm on a full frame. The answer to your question is a subjective one as others have mentioned; it's a personal choice. I shoot both 50 & 85 but for serious portraiture and commercial work, I always use my 85.. . the silky bokeh and wafer thin field of focus work to my advantage with regard to expressive, emotive portraits & fashion work. I use the Sigma 85mm EX DG HSM on a Pentax K3. I would consider using a 50mm for more serious work but I'd only have faith in a model with a larger diameter to get results I'd be happy with....
above image taken with my Pentax 50mm auto prime 1.7
above image taken with my Sigma 1.4 EX DG 85mm
All that said, if you shoot fully on manual and know how to get the best out of your kit you'll do ok with either focal length.
50mm on cropped do not work like 85mm on full. whoever says that do not understand anything from photography or physics.
field of view may be somewhat alike.
but bokeh and background separation are much more different.
85mm bokeh and separation is %70 better on a 85mm.
I got the Canon 85mm "sharp as a tack". First time I used it, the house was bearly big enough to back up far enough. For a small family grouping, not a single close-up, it's too long.
Look at this Depth of Field calculator which is actually a full angle-of-view and distance planner. See what the lens might be good for in terms of how much room you have.
You might also see some side-by-side example of the perspective of a face shot with each length. Use your existing zooms; you're looking at the perspective not the sharpness. Amazing that some people don't even perceive that a wide angle (way up close) looks wonky. I'm always telling family members to stand farther back, don't zoom back. Point is, many people might not notice any difference as long as it's over 45mm or so. That's how a face looks when talking to someone in front of you.
For a picture viewed from farther away (that is, larger than life sized print!) It may seem odd to mix cues; it should look like a (big) face seen farther away. The normal perspective one might look more intamite, though. Context of the resulting print matters.
Note that a "nifty fifty" for Canon is cheap and has very good quality. There are a couple variations and versions you can read about. I have an original from 1987, first month of production. So you might look at the the many x cost difference, too. Get an original on eBay and then later see if you want a pricy 85.