The 85mm is just longer - there's less background showed from the
same distance as a 50mm. This also makes the bokeh look a bit better.
That's a start, but doesn't really cover pros and cons.
The lenses are here.
http://www.usa.canon.com/consumer/controller?act=ProductCatIndexAct&fcategoryid=152
50 is smaller and lighter and costs less than the 85/1.8, whether 50/1.4 or 50/1.8 II version.
50/1.8 II is very cheap, but is also cheap build quality and lacks USM (fast and quiet AF).
50/1.4 costs more, but is much better build, has USM for better AF speed and provides better bokeh than 50/1.8, but the bokeh may not matter to you.
50 gives you a wider field of view than 85, so you can capture more from closer.
50/1.4 at f/1.4 is obviously faster than 85/1.8 at f/1.8, so you can get shorter exposures for the same conditions and other settings to freeze the action better with the 50/1.5 wide open, even if the 50/1.4 is often a bit soft at f/1.4.
Being shorter, the 50 is less prone to camera shake for the same exposure times as the 85.
If you get a flashgun that supports ETTL II, I don't know whether the 50/1.4 (and 50/1.8) support ETTL II. That's probably less important to you than other considerations.
85/1.8 is a bit bigger and heavier and costs more.
85 is better quality than both 50s, especially 50/1.8, better optics and has USM for fast AF.
85 gives you a narrower field of view, so you may need to back up (if you can), but it also gets you in closer from further away.
At the same f-stop, you need a shorter exposure than with a 50 to keep camera shake under control. Not an issue in good light, but can be in low light.
f/1.8 is not as fast as f/1.4 (with the 50/1.4), but the 85/1.8 is sharper at f/1.8 than the 50/1.4 is at f/1.4, and probably not so different from the 50/1.4 at f/1.8 (although I don't know for sure). 85/1.8 at f/1.8 is sharper than 50/1.8 at f/1.8.
If you get a flashgun that supports ETTL II, I don't know whether the 85/1.8 (and 50/1.8) supports ETTL II. That's probably less important to you than other considerations.
Overall, I think the 50/1.4 is more versatile than the 85/1.8 for normal closer shooting and it costs less too. From further away, the 85/1.8 might make more sense. If you only get one, I'd recommend the 50/1.4 to start with. You can probably find one used for a better price and you can always sell it again if you decide you prefer something different.
--
Cheers from John from Adelaide, South Australia
John Harvey Photography
http://johnharvey.com.au
Canon 40D, Canon 20D & Fuji F10