I personally use Cingular and find it to have great coverage. I pointed you to TMobile because it is much easier to get SIM cards for TMo USA on eBay. Typically you have to go to a Cingular store to get a Cingular prepaid SIM and that costs about $25US.
I know that TMo recently implemented a roaming deal with Cingular 850mhz but I am not sure if/how that works with the TMo prepaid.