First, a warning about Sprint. The analogue roaming referred to costs about 60 cents per minute - and it can occur within your home area in "dead" areas.
Cingular has worked out best for us. We currently have a plan which gives us 250 minutes per month and it includes long distance, to or from anywhere in the US. The cost is $29.95 per month. Of course, no matter which company you use, the quoted price does not include the MANY extra charges added on by the government. You can figure that there will be about $10 more per month added on by those charges. These charges, however, are much higher on a per minute basis, than for the "included" minutes. Cingular does have a new plan which will allow you to "roll over" the unused minutes to the next month.
An advantage to T-Mobil is that you can get International long distance for 99 cents per minute. So far, that is the only company that offers International service (I mean that you can be out of the country and use it.) Cingular says that they will soon be offering it, but I know that if they do, they will have to require that you use a triband phone, because the frequency they currently use is not compatible with that used in foreign countries.