I tried an ATT aircard for my laptop, and the reception was terrible. My iphone would have 5 bars and the laptop with the air card would have 2 bars. I took it back. Folks have told me that they have better reception with the Verizon air cars (in the Gulf Coast- beach area). I looked at the Verizon aircard, it is the same 3" X 1" usb device that ATT uses. I'm worried that I will have the same poor performance from the Verizon aircard. Anyone have experience using the aircard in the Gulf Coast onshore/offshore areas?
As James said, it's the network, not the card. And also the location. A Verizon card is different internally from an AT&T card because they use different technologies (EvDO for Verizon, HSDPA for AT&T). But both are comparable under the best-case scenarios for the two carriers. I have both an AT&T aircard and a Verizon MiFi. In side-by-side tests the AT&T slightly outperforms the Verizon when both have a good signal - I can get 1.5 Mbps on the AT&T, and 1.4 Mbps on the Verizon. The BIG difference is that Verizon has more "best case scenarios" than AT&T does. Where EvDO and HSDPA are not available Verizon and AT&T both fall back to older technologies. In these cases AT&T performs better generally. But again, Verizon's EvDO coverage is better than AT&T's HSDPA, so you will fall back less often.
Now to location. What matters is not which is better on average, but which is better where you will be using it. You have a better chance with Verizon because the network is broader overall, but you should verify that it works where you need it to a level of performance you find acceptable.