I'm on Verizon, and one of the good things about them is that V to V calls are totally free. AND Verizon has both pay-as-you-go and contract plans. Pay-as-you-go has much less service area though, and you'll have roaming unlike Verizon's no-roaming EVER contract plans.
Plus, I have signal in places my girlfriend's Nextel phone didn't, even if it was just a bar or two more.
Oh, and PLEASE FOR THE LOVE OF GOD stay away from Nextel!!! That damned annoying 2-way beep is like ARRRRGH!! Although you might like the fact that they won't turn your phone off until your bill exceeds $2000. (long story)