GSM has poorer penetration into buildings, and Cingular's GSM network is not comprehensive -- there are gaps.

Verizon has the best coverage in the US, but is also the most expensive (and focuses on gimmicky phones so that it can charge for video-over-cellphone, still picture transfers, ringtones, etc.).

If you're concerned about coverage (and not a teenager), you should be able to find an affordable Verizon plan that meets your needs. Your Cingular phone will not work on Verizon's system.

Note that Verizon is pushing their all-digital phones, but still has CDMA/analog phones available. These are what you want if maximum coverage is desired -- there are still places out in the sticks that are analog-only.