I don't think Sprint will go to GSM. They are CDMA just like Verizon is. So you have T-Mobile and ATT/Cingular (merged) as your choices. I think ATT/Cingular has much more coverage in the USA than T-Mobile does. Just check out their coverage maps on their web sites. As far as customer service is concerned, I am not sure who is better.
Does anyone know if Sprint is going to switch to a GSM service soon? If not who has the best GSM service out there? ATT, verizon, or cingular?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic