I've used both and by far my choice would be T-Mobile for the following reasons:

- plans are MUCH cheaper
- customer service is MUCH superior

I have not been anywhere (with the exception of the basement of a building in downtown Manhattan) that my t-mobile blackberry did not work.

I have to admit that I am sorry to see ATT go down in flames the way they have. I was with them for years, starting with my first ATT "one rate" when they first started it, but IMHO they have forgotten who their market is, what customer service is, and how to compete and they are now just trying to survive based on their brand name....