Definitely consider Verizon. We traveled out west with some friends last year. We had Verizon, they had AT&T. Their service got turned off after using just a fraction of their regular monthly data plan. When they called AT&T to find out why, they were told that if they read their contract they would see that although they may have a data plan with 6 GB data, if they roamed off the AT&T towers, they were limited to only 200 Mb. Any additional usage would turn off their service.
I was shocked to learn of that so I immediately called Verizon. They assured me that Verizon did not do what AT&T did. We continued to have service throughout the trip without interruption.
No matter which carrier you choose, call them and ask how they handle "data roaming" which may be handled much differently than "voice roaming".
There were a few places where we didn't have service and a lot of places where we only had 3G service.