They both work with the Blackberry phones. A friend uses T-Mobile with his 8800 and I have AT&T with my 8300. I get reception when he cannot, but not vice verse. I used T-Mobile several years ago and found their coverage spotty in the cities I was visiting at the time (CLT, SFO, and SJU). Cingular/AT&T had great coverage in those cities. YMMV