I used to use pre-pay in the past and have had both Trac Fone and Virgin Mobile.
Trac Fone has very very good coverage, in fact their coverage rivals many "regular" plans. But their cost per minute tends to be expensive even for pre-pay.
Virgin Mobile is among the least expensive pre-pay plans when you calculate on a per-minute basis, especially if most of your calls are over 10 minutes long. But their coverage is your basic "cities and highways" coverage (though this does cover almost any major highway or mid-size and above urban/suburban area).
So if you live in or find yourself in rural-ish areas often, you're probably better off with Trac-Fone. Otherwise, you'd probably be best with Virgin Mobile. There are other pre-pay plans (T-Mobile's "Easy Speak", Verizon's "Go Plan"), but Trac-Fone is in my view the best for coverage and Virgin Mobile for costs.
Whose towers/systems do TracFone and Virgin Mobile use? What would be a better phone for a 13 year old?