As noted above - Game Breakers is a great shop. A true hobby hotspot, where you will meet people who really love the hobby on both sides of the counter.
Miraj does sometimes have lower prices on certain things, especially after release, but it depends product by product.
I personally really like Jim's as well. I find the guys there pretty freindly.
Another good spot, over on the Quebec side is Coin de la Carte Sportive. It's not far from the museum of civilization, almost across from the St. Hubert Chicken place. It's another spot with great atmosphere. A bit smaller than gamebreakers, but still a good shop.