Have never been to the Marriott but the Westin is fine, typical Westin. Close to everything downtown; most importantly, Primanti Bros. Long walk to PPG though

If you are a Hilton person, there is a Doubletree right across the street from PPG. I don't know if it is good or not, but I would love to try it as I lived there for several months before it was a hotel. I went to school in the building that is now the Marriott.