35L to pair with the 85 would be my choice, though if I was asked to take only one lens to an assignment, it would be the 50L.
Everybody is different with respect to how they see and what focal lengths "feel" right. For example, 50mm works for me and I've always loved the 35mm lens. On the other hand, I find 28mm awkward. If I'm shooting with a 28, I always either: a) want to crop in to bring the composition more in line with a 35mm lens; or b) wish I had a 20mm or 24mm for a more dramatic perspective.
If possible, I'd try to rent or borrow these focal lengths to see which fits your shooting style best.
I purchased my first 50L in 2007 and I sold it. Then I got 24L, 35L and 85L. Well, couple of days ago I bought 50L again and I can tell you I won't be selling this one any time soon. IMO, it all comes down to personal preference and I like 50L better than any other L prime. Maybe is just the focal length that works for me. Anyway, don't base your decision on silly feedbacks people leaving for 50L, this is an awesome lens. As previous post states, I would also suggest renting both lenses and then decide ... this might cost you $100 but you'll pick the right one for your style of shooting.
