The Hilton is a bigger dump than both. The rooms are small and old. I believe they are in the process of renovating some of the rooms, but I'm not at all a big fan.
Apparently the Renaissance (Mariott) is very nice and it is in a great location. I have never been inside, but I hear it is nice.
You could also try the Omni. I don't believe there is a Hyatt in downtown Pittbsurgh, but then again, I'm not sure.