The company with the closest tower usually wins, all other things being equal.
However, if a USA account is being used in Canada, and a Canadian add on is in place, coverage may be identical.
Also, in Canada Bell and Telus have a tower sharing agreement--so there is identical coverage. One more tidbit is that in Saskatchewan, Bell and Telus piggyback on Sasktel towers. Data through put is another issue and at least in Saskatchewan Telus and Bell don't get the same speed as a native Sasktel device. These rules were Federally mandated in Saskatchewan.
