If you're skiing, it's a close call between Mountainside and Summit Watch.
If you're not skiing, it's Summit Watch because it's in the heart of main street.
Positives of Mountainside: Slopeside access. Tremendous pool/hot tub area. A short bus ride from everything. Negatives? Much smaller rooms from Mountainside, not really walking distance to anything but the ski area and some retail associated with it.
Positives of Summit Watch: Huge rooms, great location where you can walk to a lot of food and shopping, a very short walk to the Town Lift (less than 50 yards IMO). Negatives? Pool and hot tubs are only OK, the return run when you're skiing home to the town lift can get icy and is disliked by beginners and some intermediates.
Just wanted to add that rooms are much better at Summit Watch. Some are over the street (never really noticed noise) or some face the inside shop area.
I love Mountainside for the pools & hot tubs, exercise room, and steam room and sauna in dressing room. I hate that some rooms have no view. The rooms are ground level, and only have a view of the hill - close-up and personal.
But Westgate is so far from downtown. It's so nice to walk around Park City, with the shops, and theater, and restaruants. There's lots of places to sit outside at Mountainside at Park City Mountain Resort.
You didn't mention what time of year you are planning on going.