I have not had many coverage issues with AT&T. I think that is highly dependent on where you live. I hang around mostly in Texas, Hawaii and maybe California from time to time.
The big thing when I first got AT&T a long time ago was being able to use the GSM system. I travel overseas a lot and AT&T phones worked just about anywhere. I've lost track of where that stands with newer Verizon phones, but a few years ago you had to rent a phone for international use, and when the first international phones came out they had pretty poor coverage because they didn't use GSM.
The other thing I prefer about AT&T is that I can talk and use data at the same time. I can have a headset, be on a call and still use maps, look up something online or use other apps while I am talking. I've even gotten calls in the plane while using WingX Pro. This is only relevant for the iPhone. It doesn't matter that much for the iPad, but since I had a phone first, I wanted to use the same provider for all my cellular needs. It also helps that I have a really good corporate discount from my employer ...
