Our Canadian cell phones work with AT&T. They also work with T-Mobile if that signal happens to be stronger. They don't ever find Verizon, so I think our cell phone carrier doesn't have a roaming agreement with Verizon. Therefore, I can't really compare coverage. I've looked at coverage maps at AT&T and Verizon websites, but of course that would be skewed in the respective company's favor. This is why I've been asking end users, and I've noticed most people we talk to do go with Verizon.
As an aside, we noticed on the way south that AT&T seems to have abandoned a large area of Montana. Our cell phones picked up a network called "Sagebrush". No roaming agreement with our Canadian cell provider, so we had no service at all for a few hours. Found a discussion about this here:
http://www.howardforums.com/showthread.php/1849919-Coverage-changes-eastern-MontanaIt was pretty bizarre crossing the border and having no service in an area where we had always had AT&T. Other than that, we've had good coverage with AT&T, but, as I said, most RV'ers we've spoken to here and in person do recommend Verizon.