First of all, international roaming from U.S. carriers is expensive. But, the one thing that Sprint and Verizon did not tell you is that international roaming is very poor with them. To use their roaming in most of the world you have to rent phones, because their phones just will not work.
Cingular and T-mobile use
GSM technology which is the world wide standard. Only a few countries do not have it. So, with international roaming on your account with Cingular or T-moble you can call from most of the world.
Two side notes on this, you have to have phones that will operate on the frequencies in the countries you are going to, meaning quad band phones, and it is then cheaper to get the phones unlocked and use a prepaid
sim while over seas.
As far as coverage in the U.S. goes, Cingular provides the best of the two, but it really depends upon where you are.