Are you a beginner/intermediate photographer considering your first prime lens for portrait photography? The 50mm 1.4 or 1.8 and the 85mm 1.8 are all reasonably priced, "entry-level" lenses that can produce fantastic results. Both Canon and Nikon have these lenses in their respective line-ups. So which one to choose?
Just like everything in Life, it depends on the situation. In my opinion, the 50mm is going to work better on a cropped sensor, because it will it behave more like a 75mm. The 85mm is beautiful on both full-frame and cropped sensor SLRs.
So, just buy the 85mm, right?
Well, it depends. Let's say you're at a restaurant, celebrating a special event and you want some candid shots in low-light. In this case I like the 50mm 1.4 because the 85mm is too long, unless you're shooting full-frame but even then I feel the 50mm provides more flexibility. And that extra 2/3 of a stop can actually come in handy at night in a bar or someplace...
The 85mm is unbeatable for a day at the beach or the park, however, because of the longer focal length. Your subject is typically further away from you so the depth of field wide open at 1.8 is perfect for capturing fleeting moments of your kids running around. Whereas in this situation I find the 50mm too wide and the DOF too shallow (at the closer distances) to get consistently sharp results, especially on a moving target.
I'm talking Nikkor here, cause that's what I shoot, but I can't image Canon equivalents being all that much different:
50mm 1.8 ~ $150
50mm 1.4 ~ $350
85mm 1.8 ~ $500
Honestly, at 1.4 the 50mm is so hard to get focus that I rarely open up all the way. It's nice to have, and the build quality is slightly better, but you could buy 2 1.8s for the price of one. I don't think you'll be disappointed with the 1.8 in either focal length.
That said, I'm very pragmatic with my gear. I'm NOT a gear-head at all. I had a tough time choosing between these lenses and in the end I've gotten both, but that took a couple years ;) Hope this helps you, even if you end up doing the same!