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.