Not owners of either MS or SW, but we have visited Park City 6 times during ski season, trading in and staying at each resort 3 times, and we love both. For your October week, I'd MUCH prefer the in-town location of Summit Watch, and I feel strongly about that. Main reason is you can walk to dozens of restaurants and shops, and it's simply a far more convenient location. The town is so walkable, and quite fun while you're doing it. Plus, anytime you wish to go anywhere, such as a grocery store, it's a much shorter trip via the free city shuttle buses, too. SW units are larger, and you'll feel it.
REQUEST a unit whose window view faces the side of the mountain where the Park City Ski Slopes are located, not the rear side where the highway is......big difference.
Mountainside Resort is fantastic, but best as a ski-in/ski-out location. The village around MS may be sort of dead then, too, I'm not sure, as most of the shops are ski-related, and perhaps even closed for the season. Just a thought. There is a good Mexican restaurant there called Baja Cantina, tho. Really fun.
As an aside, I'd highly recommend that you visit the restaurant Zoom, just adjacent to Summit Watch at the upper end of the resort near the indoor pool. You'll know exactly where it is anyway because it has the most heavenly smells------it's our favorite restaurant, and owned by Robert Redford---it is superb in every way. Also highly recommended are Riverhorse on Main, and Cisero's (Italian). Sooo many great spots in Park City......it's our favorite ski town by far. great great town.