AT&T has spotty 3G coverage depending on where you live. I live in San Antonio, TX, which is the ex-headquarters of AT&T, so my coverage is nothing short of awesome. If you're on the extreme west or east coast, your mileage may vary since AT&T may not be the predominant carrier there.
Also, take into consideration your "moral" views on phones. Are you the type that gets all in a tiffy over "open-ness" and has a problem with Apple's strict content control on their app store? If so, bypass the iPhone.
With that being said, I love my iPhone(s). I'd be willing to bet you would, too... it is most definitely a "cool phone." Why would you leave Sprint? I don't think anyone but you could possibly know the answer to that question.