I've had two ATT phones and traded them in for a T-Mobile about 8 months ago. T-Mobile is the only way to go if you travel internationally. They are one of the few if not the only US company that will unlock your phone so you can use local SIM cards or use the International Worldclass Service (much less expensive than other US competitors and better coverage).

In Seattle the coverage I've found is on par or better than ATT. No dropped calls or the trouble that channa mentions. I would try out T-Mobile and see how you like it..you can always return it and get something else. If you don't travel internationally ATT or Nextel would be fine I'm sure. Good luck.