Sprint has better service and costs less.
We switched because I wanted to buy an iPhone, which I still haven't bought . I wasn't as wise as this writer:
ATPM 14.04 - Bloggable: Shallow Depth of FieldMy friends weren't as forthcoming, but I can personally confirm that in the midwest AT&T drops calls routinely. My phone may show "four bars", but that's just a little AT&T joke. I go from four bars to no carrier in an eye-blink.
...Then, there’s writing about the iPhone. You see, I don’t have one, because I don’t have AT&T service. And until my friends who do aren’t constantly cursing dropped calls, I have no intention of switching carriers and buying an iPhone...
Cost? AT&T costs us about 70% more than Sprint for a similar set of services. I think this huge gap is partly an artifact of our usage patterns (two phones, moderate voice use, frequent calls to Canada, very little roaming on Sprint) but I suspect AT&T would cost most people about 20% more than Sprint.
Contracting trickery? Sprint has a nefarious history of covert contract extensions, but they've been getting better since being sued in Minnesota. AT&T has the rebate scam from Hell.
Sprint wins across the board on voice quality, cost, and contracting. I'm amazed.
The only thing AT&T has is the iPhone, but that's a very big thing. If you aren't going to buy an iPhone immediately, however, don't switch to AT&T.
As a current AT&T customer, I join the world's pleas for a great Google Android phone, and for a future iPhone free of AT&T.
 The timing of the switch was dictated by the impending death of my wife's beloved Samsung i500, I was waiting to see the shape of the SDK before committing to the iPhone. The SDK took so long to be revealed I ran into the pending iPhone 2.0 release!
 The contract lock-ins, switching costs, and the pricing costs all promote bad business practices.