IMO, the 50L is as sharp as the 85L, but the 85L wide open wins at the edges and corners. By F/2.8 it is much of a muchness.
However the strong point of the 50L is that both its foreground and background bokeh are very good, and that it is extremely flare resistant. The good fore- and background bokeh make this a lens with extreme pop (check some of the photographs in the 50L sample thread, especially those by Wem), provided you have something in the fore- and background that is OOF. With the 85L the background is especially nice, the foreground less so. Also, the 85L has an interesting feature, namely a halo around the subject, which becomes (clearly) visible under specific lighting conditions. If you do any fancy PP, it becomes very visible, and this is not necessarily a good thing.
However, I use both, and like both very much, although the 50L spends most time on camera
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Oh, BTW, both lenses do have focus shift issues. On a good and well calibrated copy this is within the AF constraints of a fast lens (1/3 of DoF accuracy), so not a field relevant problem. My 50L has focus shift between F/1.6 and F/2.8, reaching its maximum at F/2 of approximately 1/6 of DoF, and the 85L II is similar, with a maximum shift of 1/4 of DoF at F/2, in both cases at close focusing ranges.
Kind regards, Wim