All of the carriers share the towers, but the tower owner (usually Telus) gets the top spot. Bell usually bids highest and gets the second spot, Rogers next and Mobilicity and Wind are usually lowest. Mobilicity and wind also have microcells in some dense areas in Vancouver too which is why they have better coverage in places like Metrotown but sketchy coverage away from dense areas.
Last time I checked, Telus, Bell, and Rogers had nearly identical plans and you had to go to the rebrands (Fido, Koodo, Virgin) or small carriers (Mobilicity, Wind) to get any differentiation, Sadly Mobilicity and Wind have roaming-only coverage on Vancouver Island, so they are ruled out in my case.
Also of interest in some cities in the province.... If you are a shaw internet customer you can connect your phone to the "Shaw Open" wifi access points and once you register the device (you need your shaw email address and password), it will be remembered so you can save on your data plan and get wireless-n speed. You can register up to 5 devices per account. I have "AutoSync" on my phone which toggles my sync when connecting and disconnecting wifi so emails come in pretty steadily as I drive past the access points.