We have both AT&T (personal) and Verizon (office) in the Princeton area. We get pretty good service with both. Verizon is a bit better (but then you have limited selection of smart phones unless you like the Blackberry). We get some dropped calls with AT&T, particularly around the Princeton Jct. train station.
The only info I have about T Mobile is a friend who moved to Princeton about 8 years ago. He got crappy service and switched to Verizon. But that was years ago, it might be better now.
And yes, it is a good question.