As a Sprint user I'm looking forward to my upgrade later this week to an iPhone.
Sprint's return policy gives you 14-days with no restocking fee. Remember, the day you ORDER it counts as Day 1, even if you buy it at 11:59 PM. So, make sure if you truly aren't happy that you return it on around Day 10 to be on the safe side.
AT&T's return policy I believe gives you 30-days, but there's a $25 restocking fee per iPhone.
So, you can try them both for the cost of $25, plus your data plan.
I've found Sprint is pretty good in my area near New York City... I'm able to Stream Pandora on my Blackberry Curve on my ~20ish minute commute with out any interruptions, and I very seldom have a dropped call. Once every few months.
I honestly don't know what the big deal is about the Sprint data speeds. The only thing you're using that speed for is to race closer to your data cap. Incoming overages and/or throttling! As long as your phone can do whatever it is that you do without buffering or stuttering, then who cares? If the content you're viewing utilizes, say, 250kbps down, what difference does it make if I'm on AT&T or Sprint or Bob's Cell Phone for that matter? I don't really need to download a 1 GB App in 10 seconds. And if I did, I'd use a Wifi hotspot.
To each their own.
Try Sprint, it's free.