If you have an AT&T phone that does analog you already have more coverage than Sprint. Sprint has the most *digital* coverage compared to anyone else's digital network. Sprint also offers analog roaming, either at extra cost per minute or at extra cost per month with that roaming included, assuming you get a phone that's digital/analog, but unlike AT&T and Verizon, they have no analog network of their own. (Sprint's network began later and they never competed in the analog market.)

AT&T is a bit in flux due to their switching from TDMA to GSM setup. If you have the GSM-based AT&T service, you would actually get more coverage area by switching. You might want to look at Verizon more than Sprint though.