You should go to Whistler for a few reasons:
First, if you want to drink, Whistler is the only place to do it without having to fear getting an MIP (minor in possession). Secondly, the nightlife in Mammoth is not great at best. I lived there all last season, right in town, and believe me, it kinda sucks. There are a few bars, but overall Whistler kicks Mammoth's butt in terms of night life at any time of year (I am within driving distance to whistler, so i know both places). BUT, the biggest reason that you should move to Whistler for next winter is LA NINA!! This next winter is supposed to be a La Nina winter. Let me recap a recent La Nina for the northwest and canada: WORLD RECORD SNOWFALL. One word: Baker. Baker is situated right near the Canadian border and the last time a La Nina cam ethrough they got over 110 feet or so of snow. On the otehr hand, California sucks for La nina. They have a much drier winter as well as warmer. They got much less snow than normal.
In conclusion, you should definitely go to WHistler!!!