I am biased but I don't think the two really compare in terms of skiing.

Everything at Whistler (and from the Westin) is walkable and practically ski in and ski out (less than 50 meters walk). The skiing is to die for.

Beaver Creek, although an excellent ski area, is much smaller and much less challenging. You'd likely want a car to get too and from the other ski areas in the corridor, which negates the value of the ski in ski out principle if you are traveling between areas.

I haven't stayed at the Beaver Creek property but the Westin Whistler is pretty solid.