If you are planning on buying prepaid service, T-Mobile wins, hands down. I have had my prepaid account by them since 2001 and have long since been put into their bonus rewards program. Their range of voice call costs anywhere between $0.32/min for $10 refills to about $0.06/min per $100 refill (in/outbound domestic U.S. text messages are a flat $0.10/msg).

Plus the best thing about T-Mobile ToGo is that this is the only U.S.-based cell carrier that allows it's roaming customers to roam internationally upon activation of account ... with no credit or background check of any kind.

I also have AT&T GoPhone prepaid. Unlike T-Mobile ToGo, GoPhone accounts can't roam internationally.

In terms of coverage, GoPhones can only access certain cell sites owned by AT&T that are site designated as a GoPhone accesible site, which means that your prepaid coverage is much more limited than regular AT&T post-paid. T-Mobile ToGo, however, can pretty much access any GSM cell site with no problems so their coverage is vastly superior.

Lastly, T-Mobile ToGo has a program called Gold Rewards that gives more minutes per refill once you're reach $100 worth of refills. Plus with that rewards program, once achieved, *all* refills have expiration dates of exactly one year since last refill. GoPhones also have a similar program, but the best term of retention period to keep the phone active is 90 days with a $100 card.