You've got a tough choice here. Personally, I think the 85mm will give your portraits with a little more "pop" to them (I own a 50mm and a 100mm and prefer working with the 100 if I have the space to back up). That said, you've already got an 85mm in your 70-200; you might want to play with that as a portrait lens and see what you think. The 50mm is a fine lens, and it's noticeably different than the 35mm-- it would fill in the gap between lenses. You can also get closer to your subject, which changes the mood/intimacy/connection.