Can't speak about Bridger as we haven't been there for awhile, but the last 2 seasons we were at BS over Pres week. I've been told that is the busiest week of the year, and as already mentioned, aside from the tram, lift lines are pretty much non-existent even on that week/weekend. Moonlight is worth checking out as well, and had very light crowds on Saturday (or was it Sunday) pres weekend. We skipped it the first few seasons it opened, but now make it there a few days during our annual BS week (cheaper tickets definitly a plus as well).
When at MSU, ask about discount tickets for the ski areas. My niece did the tour a few years back and got some. I have a HS senior that should take advantage of that as well when we go for our ski week, but he doesn't want to cut into his ski days while out there. Otherwise, Costco in BZN usually has BS/Moon tickets.
Speaking of crowds, I'm trying to decide between a Christmas to New Year's ski trip through Montana (Whitefish to BS), or to SLC. My kids have yet to ski Utah, and it's been 20 years since I have, so I kind of want to go that route. But...we are really spoiled never having any lift lines to speak of in MT. Any thoughts on crowds over xmas at SLC areas?