I was on Sprint for a long time, and am now on Verizon with whom they share. It's pretty much unbeatable. I got 3G coverage in random corn fields driving from Ohio to Cali. And anywhere even remotely civilized, never a dropped call or lack of blazing 3G.

I don't know anything at all about the merits of CDMA vs. GSM on a technical level, but as far as coverage in the USA it's not comparison.