There are two situations in which choosing AT&T for your data makes good sense: if you travel internationally, especially to Europe, and if you want an iPhone. If you don't fit either demographic, you'd be better served with a Verizon Wireless subscription.
Whether or not you'll like Sprint's network depends greatly on where you live. Compared with AT&T, Sprint can seem fast and powerful, but only if you're in one of its sweet spots. Sprint's Mobile Broadband falls well behind Verizon Wireless in coverage and reliability. All in all, it's a good choice, but not the best.
If you need wireless data in the U.S., Verizon Wireless is the best choice. It has the most comprehensive 3G network, and it wins awards every year for coverage, consistency, and call quality. Verizon's rates aren't the lowest, and the company may not carry the latest, flashiest devices, but when Verizon says "It's the network," it's not just boasting. This network has no equal.