Both are in the same price range, used. However, the question is should I sacrifice the focal length or the f-stop? I would primary used the lens for portrait... and looking for something that would really give me a good depth of field.
Overall without considering focal length I think the 85 is the better lens. It is sharper wide open and has a true USM focusing motor. The 50 1.4 is good as well but the micro USM focusing motor is known to have issues. I recommend always using a hood with the 50 to minimize the chance of something hitting the front of the lens and damaging the motor (lens extends during focus). The 85 does have some issues with fringing at the edges of high contrast objects.
It's not easy to find a 50 that's sharp wide open (though not impossible), while it's hard to find an 85 that's not. The longer focal length increases isolation on shots taken at distance, IME, equalizing DOF on most but the closest shots. Crop or FF?
I have and love both.
Assuming its used on the 60D in your profile i would choose the 50mm for portraiture. But in that capacity i wouldn't use either lens wide open as DOF is too short.
The 85mm is a lot better. If you have a crop body though it might be really tight. I used mine on a 20D back in the days and it was for headshots mainly.
Both do a fine job blowing out backgrounds. Mechanically, the 85/1.8 is a much better lens. I've had two 50/1.4s and two 100/2s (cousin to the 85/1.8). I sold my crop gear specifically to use the 100/2 at native coverage. It's that good, and the 50/1.4's focusing mechanism is that questionable. Consistency of the first 50/1.4 on a 40D was marginally good after two trips to Canon. The replacement new copy, just marginal, and on a 7D, terrible. I didn't see any purpose in keeping an f/1.4 lens I wasn't comfortable relying on at f/1.4.
But let's say you're willing to roll that dice. The 50/1.4 has strong perspective on a crop body. The 85/1.8 will give you flatter, more flattering images, but it'll be long for indoor use, harder to handhold, and less intimate. If you're uncomfortable with the 85's working distance, the other advantages won't amount to much.
Go with the 85. It's better built, faster focus, and also sharper. I noticed that the 50 f/1.4 (at least the one I rented) overexposed by at least 1/3-1/2 an f-stop, which was very annoying. It also had trouble with the AF "hunting" even in good light.
as an owner of both of them, personally I think the 85 is the slightly better of the two although it does seem to suffer a bit more from colour fringing than the 50. That can be easily corrected though and I've found mine to be sharper. Plus the shallower DoF of the 85 is nice. Despite all this, 50 is a better focal length for me a lot of the time so I use that more. I think chose the lens that gives the focal length you want.
I ended up selling my 85. The 50 is one of my most used lenses. All of this demonstrates only that everyone has different tastes, styles and needs. Both are fine lenses, though more users report problems with the 50 (some are soft wide open and the focus motor can be damaged by an impact to the front - I suspect these problems are relatively rare though).
