As you have already have the 15-85 a 50 would go nicely. I have the 50 1.4 and am very pleased with it. Although not amazingly sharp at F1.4, it produces excellent razor sharp images from F2 and is much better built than the 1.8 model.
(If you do get one i recommend leaving the hood fitted at all times to protect the focusing assembly which i have heard of being broken a few times!)
Good luck!