I would say Sheraton
I am from Pittsburgh and while I have stayed at the Sheraton and not the Westin, I would say the Sheraton simply because of location. The Sheraton is at Station Square (a nice area of restaurants and shops) which is a more pleasant place to be at night. The central city is not exactly the nightlife mecca and many places close. The area around the Sheraton is better to stay. You are close to the inclines to go up the mountain, you can walk across the bridge to downtown, or can take the subway to get there.