In my experience (as a domestic user) both AT&T and T-Mobile can be acceptable or can suck, depending on exactly where you are in the country, and sometimes even within a city or suburban area (for example, in the city where I used to live T-Mobile sucked at home, but was great at work, while AT&T was exactly the opposite). Those nice homogeneous-looking maps of "coverage areas" are lies, to a very large degree. :-)
If you're traveling for work, maybe ask the people you're working with which carrier is best at their site.
If you're traveling for work, maybe ask the people you're working with which carrier is best at their site.