Get whatever suits you best in terms of focal length. I rented both of them, and ended up buying both of them. They're both THAT good, in their own way.
The 85 is clearly the better portrait lens. Sharp at 1.2 (though sharper by 1.8), and much nicer background compression due to the focal length.
My 50L gets the most use though, but mostly for its a more widely applicable lens. Its close in sharpness to the 85, but you don't get as much background compression (which can be a good thing, as it keeps the subject in context of the background.) 50mm is a very 'convenient' focal length on a full frame camera though. 85L is kind of long...it can be used indoors, but is troublesome if you don't have a lot of room. Plus its rather massive for a prime.....The 50L is very compact and easy to shoot with, and you get a lot of the same quality/look of the 85L.
The 85L is my go to for serious portrait shooting. The 50L is my 'daily driver', and my main walking lens.
3Rotor wrote in post #15476943
This may sound crazy but I have yet to mount my 85 to the 5D3, I've already had the 5D3 for about 4 months now, it has been glued to my 70-200 since day one. I'll have to give it ago and see what I've been missing out.
Give the 85L a chance. The 70-200 is great, but when shot wide open, the 85L's sharpness and ability to isolate the subject (without completely blurring out the background), combined with its beautiful bokeh, create a sense of dimension that the 70-200 can't.