Hola! Don´t go to LL in January, it is too cold, the days are too short, do go in spring (March-May), we're planning on 3/8-22, 2008.
Alta is great, but not at all like Vail (no town, no nightlife, hardly any restos.,etc. Just plain GOOD skiing! On the other hand, if you stay in Park City you can take the free shuttle to PCMR, Canyons, and Deer Valley. Park City si a great little town, has everything you want.
BTW, we enjoyed Cerler a few years ago, better than Baqueira, And Sierra was a blast as well!