The company I work for keeps a pool of aircards for employees who travel (mostly sprint, but we also have two verizon and two at&t). I took one from all three providers on a trip to run my own speed test about a year ago - tested them on a drive up from Las Vegas through Los Angeles, Monterey, San Francisco to Sacramento. The Sprint was consistently the fastest (I'd usually get download rates around 1.2-1.4mbps - did not test for upload) and seemed to get the best reception in most places (particularly while on the move and not in a major city, it would still remain on their 3g network). The AT&T was the true speed king, topping out around 1.8mbps downstream as I recall, but only in the middle of Los Angeles and San Francisco - anywhere else, it had spotty connectivity and would frequently drop down to the EDGE network. I never got past 900kbps on the Verizon and it also seemed to drop to "2.5g" quite often. I now carry one of the Sprint aircards with me at all times, and you'd have to pry it from my cold dead hands.
One other thing I found - not all aircards are alike. I tested through 4 of the Sprint aircards from the same location and found that two of them were consistently much faster than the other two to the tune of around 250kbps on average. All were the same model (Novotel EX720).
Testing was all done on my Macbook Pro.