This is Montana. We know how to plow and sand roads.
US 2 is the only major road that crosses the northern part of the mountains, it gets a decent amount of use and is pretty well taken care of. 93 is fine as well.
Honestly, I'd say that Big Mtn is the best value in the state. Great tree skiing, fun groomers, a decent to legit park depending on what's set up, and powder that sits for days if you know where to look. Plus, it's not as expensive or touristy as Big Sky, and is closer to the town (fifteen minutes from Wfish as opposed to an hour from Bozeman).
If you do head to Big Sky, don't get a Lone Peak pass. Super expensive, and there's no way that you can ski that much terrain in one day; the crossovers alone take three to four lifts. Just take day at Moonlight or day at Big Sky. And go up the tram at least once for the hell of it, more if the snow is nice.
Also, if you are driving from Manitoba, don't do all that driving for Great Divide or Showdown. Either one would be worth a day, but go to one of the larger places for a few.