Mont Tremblant has 92 trails and 610 acres of skiable area. It is designed primarily for the intermediate or advanced skier, with over 50% of its trails marked as black diamond or double diamond. It is about 90 minutes from Montreal.
Mont Sainte-Anne has 56 trails and 428 acres of skiable area. It has more trails available for the beginner to intermediate skier. Only about 30% of its runs are classified as black diamond or double diamond. It is a 30-60 minutes from Quebec City.
I have skied both hills and at the time, I enjoyed my trip to Mont Sainte-Anne much more than Tremblant, but I am just a casual skier, not very experienced, and I liked the views from Mont Sainte Anne down to the St. Lawrence seaway more than the trees and mountains I saw at Tremblant. It may also be that I was with a group and we partied in historic Quebec City that evening.
The advantage that Tremblant has is that there is a small village with restuarants and shops right at the base of the hill, so after finishing a day of skiing, you can relax at a spa, go for a fancy dinner, do a little shopping, then go to a hotel or come back home.
At Mont Sainte-Anne, they did not have much around the area when I went there, other than the actual skihill clubhouse. that may have changed though, as I have not been there in several years.
I guess which one is better depends on your skill level and proximity. If I was visiting Montreal from out of town, I would definitely be skiing Tremblant. If I was in Quebec City, I'd do Mont-Sainte-Anne. If I live in the area and challenge is more important, I'd look at Tremblant over Sainte-Anne.