Also it depends on what you plan to use the Cell Phone for. For instance, my wife is a home health care nurse, and has an Alltel pre-paid plan for emergencies - we pay ahead of time for a set amount of minutes, and as long as we pay $20 every two months, we accumulate all minutes and don't have to worry about overage fees, or roaming fees, as it just comes off of the pre-aid minutes (it's double-billed if we do roam, but since everything in South and North Carolina is considered non-roaming, it's never been a big deal.)
So the things to consider:
-purpose of the phone (mainly emergency use, or consistent use)
-densest converage areas of the provider in the areas you mainly plan to travel
-roaming charges for the areas you may be in occasionally