I switched from ATT to Verizon so I could have the STorm, however have been with ATT and travel all over the country for business and have never had a problem. So far no problem with Verizon either.
So guess I really don't have a preference at this point, it is more the phone offering. I think ATT has a better selection of phones, but the don't have the STORM.
I have both (honestly; the Storm is just for work) and I can, also honestly, say that I'd get rid of the Storm in a second and keep the iPhone if I had to. Service has been great with both of them so that is not a consideration. On a level playing field (signal strength for example) anyone that has used both and says the Storm is as easy to use as the iPhone is not being objective.
I've had both & hands down Verizon is a better provider in the Northeast US. I'm on the road traveling for work & can't remember a call being dropped on Verizon. I've had a few different phones with AT&T & was constantly losing service.
A few close friends are with AT&T now, and they drop calls with me almost every time we speak. I have resorted to a "text only" relationship with those people.