Depends on where you live. Coverage from these carriers varies from area to area. So maybe if you told us where you live, that might help.
Also, I think your phone options are limited when you choose pre-paid, no? What phones are you deciding between?
Im getting a prepaid plan and have two questions. First, what GSM service is best, Cingular or T-Mobile? Or would it be better to get Virgin Mobile? And which phone should i get on that carrier?