I think I have tried every "major" hotel in Pittsburgh and from my experience, they all have some major drawbacks. I concur that Hilton is as stated above. Omni has a very grand lobby and very old fashioned rooms. If that's your style, you'll like it. The rooms are small and the beds are ok comfortable. The Westin is very so-so. And not that well-located. I have not stayed at the Renaissance but have heard that if you do not have status, it is also very so-so. I have not tried the Sheraton but after learning that the rooms are recently renovated, I will on my next trip.
My advice: go for whichever one gives you the best price. It's probably 6 of one, half-dozen of the other.