As others have written, the most important factor right now is who has coverage in areas where you'll be looking to use the card/service.

Cingular's high-speed service (which is currently being deployed) is still confined to just a few markets. From a spec standpoint, it's supposedly a little faster than the current EVDO offerings from Sprint and Verizon.

I have Verizon EVDO, and I love it. Good coverage where I live and in the cities I frequent for work.

*The whole "spec" argument is one to be wary of. While the Cingular high-speed service (in 3G areas) is supposedly faster than Verizon's EVDO, when I've run speed tests on my card at home vs. my brother (Cingular 3G) and Dad (Cingular 3G) the results have come out dead even on average. Remember that while a new standard may feature a very high u/d rate in its specs, the provider has the ability to "throttle" bandwidth.