Had both, went with AT&T
Here's my reasoning: I originally preordered an AT&T on 3/7, just because I had always had AT&T and I didn't have much time to research LTE coverage, the ability to use AT&T 4G (not LTE) on the Verizon iPad, etc. Then, while waiting for launch day, and discovering that Verizon had LTE coverage in Philadelphia and AT&T did not, I got up early and bought a Verizon on launch day. That's when the fun began. Logically, it made sense that I could simply drop my AT&T sim from my AT&T ipad2 into the Verizon ipad3 and I'd be able to get the AT&T 4G (not LTE), but as everyone discovered, it wasn't that simple. The Verizon ipad3 does not recognize the AT&T sim card, so the only way to get AT&T 4g on the Verizon iPad is to first activate the sim on the ipad2, then put the active sim into the Verizon iPad. The problem is that once its in there, you still can't access it through settings. Sure, you can go online and modify the service, but if you don't want it to auto renew, your account only stays active for 60 days. Bottom line is that if you don't want to use the cellular service all the time but simply want the ability to activate it in the future, you have to either keep a spare iPad 2 around so you can reinstate service from there and then swap the sim to the Verizon iPad, or keep paying a monthly fee. Neither of these options is particularly appealing. But what really made my mind up was when I visited the Verizon store to try an iPad actually running on LTE. I did a speed test on it and it came back around 7 up and 7 down...not very impressive. With a strong AT&T signal on the AT&T HSPA+ I was able to almost duplicate that (down at least ). So I decided to stick with the AT&T. I've owned the iPad 2 since launch and have never even used the cellular option so I figure if I ever need cellular, which would be when I travel, there's probably a good chance that there may be AT&T LTE in the city I'm traveling to.
Oh, and one more thing, the Verizon display was definitely yellower than the AT&T and once I discovered that, it really sealed the deal.