My choice would be T-mobile. Customer service and billing are vastly superior. I also find voice quality superior and use both services regularly (T-mobile with an E65 and AT&T with an N75/iPhone.)

Before you do anything though, go to Target/Wal-Mart and invest $20. Buy a prepaid package from the carrier most attractive to you. Make calls and try the service out in the areas most critical to you. If service sucks, spend another $20 and buy a package from the other carrier. Dunno about T-mobile but AT&T sells the junky Motorola C137 with $10 in airtime credit for $20 at Target. Sure you have 14-30 days to return and cancel but why spend all the time doing the credit check, etc when this is so much easier? You also then end up with an oopsie phone you can shove in a drawer and use as needed.