Fortunately, I live in an area that is well covered by both AT&T and Verizon. Verizon definitely has a larger presence in the state (NC), but for my travel purposes, it's about equal.
I actually am on AT&T and have ordered an iPhone 5 on their service. That being said, I have a Verizon 4G iPad because I believe Verizon has the superior network........for pure data. I personally believe that AT&T is the better choice for voice + data. Falling back to EVDO on VZW is not an option. While I entirely admit that VZW destroys AT&T with LTE coverage, I believe the overall coverage of (relatively) high speed data (3G+) favors AT&T.
Combine that with simultaneous voice and data (I may be the only person in North America that is dependent on this, but I use it frequently), and my unlimited AT&T data, I stuck with AT&T.
Either way, I figure i will have coverage with ONE of the providers if push comes to shove. I will admit I am very jealous of that Verizon LTE coverage map, though!