There are a number of reasons why you might choose a particular cell-phone and the network it operates on, but for me, the biggest and first reason is the coverage you get. That is, can you make calls and connect to data everywhere you want to. If you live in Cobourg or Port Hope, you will be primarily interested in coverage there and only secondarily in other places like Toronto. The best way to decide coverage is to try it out in places that matter - like inside your home - but that's not always possible before you buy. For more on the coverage of each supplier, see below.
First, although there are three major Networks - Bell, Rogers and Telus - Telus and Bell have a roaming agreement so that in areas like Cobourg and Port Hope, Telus operates off the Bell network. For the small players, currently Virgin Mobile, Solo, and PC Telecom use the Bell Mobility network, while Fido, 7-11, Petro Canada, and Cityfone use the Rogers network. So for Cobourg and Port Hope, we need only be concerned with Bell and Rogers Coverage.
Click the map to expand it.
All of the Bell and Rogers towers provide LTE service.
Note that coverage is only one reason to choose - price (including bundling), service and "who you want to deal with" are other factors. But since this is primarily a technical site and coverage is technical, only differences in coverage are shown here.
The map makes it clear that coverage by Bell is significantly better than Rogers in Cobourg and Port Hope. This is especially true in the downtown areas. It's not possible to say exactly how close you must be to a tower to get coverage but it is safe to say that more towers and a tower closer to where you are means better coverage.
Rogers is currently (December 2015) planning a new site at the corner of Wilmott and King so coverage in the Lucas Point area and east residential areas will improve once that is complete.
If you never use your phone indoors, both networks will be good - it's when you go inside that the difference would be apparent. Even then, you may be OK on Rogers. That's especially true if you live in a third or fourth floor apartment, or in Cobourg near the middle of town or in Port Hope, in the North Eastern area.
For more on Cellphones and the source of the map, go to Steve Punter's Southern Ontario Cell-Phone Page.
For home Internet (wired) service information, go here.