Boise is a really cool place. I went to high school there. Bogus is really fun, and it's about a 30-40 min drive from downtown but there is not always the best snow. It is definitely what you make of it. It is where I learned to ski though, and I loved every minute of Bogus shred. Sun Valley, Brundage, and Tamarack are all weekend, or even day trip able if you are super ambitious, and all of those resorts are super fun. The party scene in Boise is not like super crazy, but people definitely know how to have a good time there. I cannot say what the scene is at BSU, but I imagine it's pretty fun, but not insane. I have quite a few friends who stayed in Boise and went there are still really dig the city and the life. I think Boise probably has more going on in the city than Bozeman does, but I don't really know. Bozeman seems like a pretty rad place and I have heard great things about Bridger Bowl, and Big Sky is way sick. I think if your biggest concern is to ski big and steep stuff, I would choose MSU.