I agree TMo has the least expensive rates but you also need to look at their GSM vs GPRS coverage maps. Data coverage is not everywhere voice is found due to the fact GPRS is an overlay to GSM. This is the same for CDMA - Sprint/Verizon. I prefer Nextel due to the fact their data coverage is the same as their voice coverage.
BTW - Sprint and Nextel are actually merging. Sprint will own 51% for tax reasons - as they are selling off their landline business.