Historically from previous releases in Canada, Apple will have the most stock and best guaranteed chance to get one, but you are looking at HUGE lineups.
The big guys, Rogers and Bell, will have a lot smaller lineups, but their stock is spotty at best. In big urban centres like Toronto and Montreal, they'll get a bit more stock, but sometimes they only get a handful.
Other places like BestBuy sometimes have barely any lineup at all, and they'll get some stock, but it can be spotty. For the iPhone 4S, local Best Buy only had about 6 models, mostly 16GB's and mostly white models.
For the 4S, Rogers also had some sort of "pre-order" system, but it was a hot mess and no-one who participated got a phone on the date.