thats a tough choice man.
mammoth has the most banger park in northern california hands down, and they are open the longest, november to july usually. North tahoe has northstar and boreal and alpine... all of which have pretty sweet parks. As far as what you should base your decision on is the quality of living. I would not want to live in mammoth, so small, nothing to do other than skiing and skating, and mtn biking in the summer. the plus to mammoth tho is the massive influx of super hot LA girls on the weekends