T-Mobile is GSM like AT&T but would require different, AWS bands for 3G. They also offer "4G" over HSPA+, which iPhone 5 just might support. However, AT&T is trying to buy them, so they might get the iPhone that way sooner or later. Sprint is CDMA like Verizon but would need access to their network. They also offer "4G" over Wimax, which Apple will probably never support because LTE will be a much bigger, more global technology. (See our wireless networking glossary for what all those letters mean.)
If you could have only one for iPhone 5, T-Mobile or Sprint, which would it be?