For example if you spend any time in west Texas, the only service that works is AT&T.
Plans change on a monthly basis and you get your best "deals" near the Christmas season. You will have to explore the coverage and the plans that will fit your travel requirements and your pocketbook.
It is an individual choice and don't let pushy sales people or "travel gurus" make your decision for you. Also, these carriers now have "no contract" plans" so changing your mind is easy if you find the one you've chosen is not the one you need.