I think that AT&T has the next best coverage. Their 3G footprint is nowhere near Verizon, but for basic phone calls, text messages and lower-speed data, AT&T works practically everywhere Verizon does. Also, if you get a quad-band GSM phone from AT&T it will work in most foreign countries (but you'll pay through the nose if you don't unlock your phone and get a local SIM card).
Sprint, except for the old NexTel service, uses the same technology as Verizon, (CDMA), but the phones are not interchangeable (due to carrier lock-downs), and they do not seem to have cross-roaming agreements. Their coverage is nowhere near Verizon or AT&T. But I think that in 911 (Emergency Call) mode they will connect you via a Verizon cell.
Boost, Cricket, MetroTel and many of the more economical pre-pay carriers are reselling Sprint service, so the same caveats apply as for Sprint.
Note that NexTel, although a Sprint product, is a completely different technology (iDen) and coverage is limited to metro areas and major highways (and surrounding areas). Outside of this they are a paperweight.
T-Mobile also uses GSM, and you can interchange phones if you get them unlocked. But, in spite of their coverage map, T-Mobile's reliability is reported to be the worst of all (except NexTel) and although the phones use GSM there is no roaming agreement with AT&T.
I currently use AT&T and my kids and I are generally satisfied.
I've used AT&T, Verizon and Sprint and my observations are:
- AT&T has the best nationwide coverage for voice calls.
- Verizon has the best nationwide coverage for 3G data.
- GSM, used by AT&T, is the global standard for cell phones.
- CDMA, used by Verizon and Sprint, is rarely available outside North America and a few Asian countries.
- GSM phones can be unlocked so you can just replace the SIM card to change carriers.
- CDMA phones can not be interchanged between CDMA carriers.
- As far as I've discovered, all the inexpensive Prepay Cellular Providers actually resell Sprint.
- Sprint has the worst coverage and reliability (as of 2004) and the poorest selection of phones
- Sprint has the worst customer service, but none of them are much better.
Coverage Maps: