Never been to Tremblant. Killington is very big, very spread out, but I think it kinds sucks for snowboarders. LOTS of very short runs (don't let their hype fool you) and they have a several different base areas which make it easy to get separated from your group -- and it takes forever to find them again. You'll do a lot of walking when you're at the bottom, lots of flats between the various base areas.
And don't let their claims of vertical fool you...that is based upon ONE very long green trail that is virtually unrideable on a snowboard. It literally crosses through a parking lot, then snakes through the woods and a neighborhood to the Skyeship parking area/gondola. It should be labeled a snowshoe trail, but Killy uses it to claim they have the most vertical in the east. Bullshit. Most of Killington's trails are very short, but they have lots of acreage, so they hype their size. And they lie about their snow conditions worse than any mountain I've ever seen. It could be 45 degrees and raining, and they'll claim it's a powder day -- anything to get the New Yorkers up there.
It's known for it's nightlife. Personally, I can't stand Killington. It's like being a tourist in Times Square. Lots of nasty, rude posers. A lot of people swear by it...I swear AT it. I won't go unless I have a really cheap ticket.
Hit up Jay Peak. Great mountain, lots of trees, and killer water park with stationary wave to surf on after the mountain closes. You can get a combo lift ticket/water park pass.
wicked now I know to hit up Jay's Peak instead of Killington bahahaha