When in the US, I found I have to disable "automatic" and force AT&T -- using T-Mobile blocks data for some reason. I don't know how to configure my iPhone to just block the provider (so I obviously have to disable it when returning to the UK or Europe).
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.
