I agree with your point about the carrier first. I see a lot of people around here switching carriers just for a phone. A lot of people went to verizon for the storm.
I don't think that's a wise decision. I would always put the carrier and the quality first.
In my area, I've had verizon, sprint, and at&t. I've had at&t for the past 6 years, did a "trial" with Sprint about a year ago, but didn't cancel at&t, well the sprint phones went back after about 15 days. Verizon, I had for 2 years, and while their voice quality was amazing, their plans were a little more expensive than what cingular/at&t were offering, and I hated how they cripple their phones. (Why can't verizon phones have bluetooth file exchange again? That's so wrong.)
For me, AT&T is the best choice. I've never personally had t-mobile. I've thought about trying them, and I may at some point, but it won't be until they get 3G in our area, which they currently don't have, so right now, AT&T is the best choice money and coverage wise, when that changes, then I'll look elsewhere, but I agree, don't switch for a phone, switch for what the carrier is going to offer you over the longterm.