Phil is right. MRG is too far out of the way. Not worth the effort unless they've gotten a good dump of fresh snow. Otherwise you'll be skiing very difficult bumps and glades on very hard snow.
When its good, it's fantastic, but when it's bad, you'll regret going that far out of your way. Getting there from I-87 and then up to Montreal will add a lot of time to your trip.
Whiteface is easier to hit if you're heading to Tremblant, and has some great steep terrain if that's what you're looking for, but it can also get very icy. However if conditions are good, I'd go there. With good snow, skiing Cloudspin and Upper Skyward on real 3,200 vertical can be a real adreneline pumper. My biggest beef with Whiteface is that their lift ticket price has become ludicrous for a State owned mountain.
If conditions in the NE aren't so great, or if you just want the easiest place to get to on a side trip, I'd definitely go to Gore Mtn.
Not as challenging as the other two, but a big mountain, with some short steep runs and usually better conditions than the others. It's very easy to make a side trip there and you can find the cheapest lodging around if you stay in nearby Glens Falls for a night.