I found that our 20" tires perform very well in the 8" of snow that we just received last week. I had no problems what so ever in the snow, and we have only the Front wheel drive. In fact, I almost thought that the traction control wasn't working on our Traverse, because I never felt that un-mistakable feeling of the wheels being slowed by the computer, and I didn't see the light on during normal driving. So, when I was clear of all others, and on an open 4 lane road, I punched the throttle, just to see if I could get the wheels to spin. To my surprise, I slipped a bit at first, and the traction control kicked in, but then it grabbed, and I took off. With that, I was very impressed that I didn't need the traction control sooner, on very slick, snow packed roads.
Thus far, I do NOT regret our 20" wheel / tire choice, with just the front wheel drive. We too wanted AWD, but just couldn't find the right Traverse with that option for us, so we "settled" witout the AWD, and so far, it's worked for us.
Good luck on your purchase, and I would guess you should be fine in most cases with the 20" wheels, and AWD, even in Canada. ;D