I have had Cingular, back in the day when it was Pac Bell on the west coast, Nextel, ATT, and Verizon.
Verizon has been the best for me, ATT would drop calls all the time and I would get busy signals on the freeways. Verizon's CDMA network at 800Mhz gives better penetration into building than the PCS signals, the CDMA system also gets rid of backround noise, may ATT phones all sounded like I was talking from the Moon when I was driving. I travled a lot for business to Michigan, Hawaii, Boston, Washington, Oregon, Phoenix, Vegas, Florida and would use my personal Verizon phone over my Work ATT phone. Go with the better Technology, CDMA is a vast superior technology to TDMA that ATT uses, eventually once they have a full GSM network it might be better but it will still be way behind CDMA in Call capacity and the features of the 1X 2.5G system. Just my 2 cents. Sprint also has the CDMA system but by using the PCS frequencies every where you need approx 2 cellsites for every one 800Mhz Verizon/ATT site, they are still a year or 2 off from having full coverage and filling in the gaps. Nextel is pretty good there are still holes in their coverage, cingular is o'k, I just know there are areas off the beaten path that do not have GSM coverage.
This is all just my opinion!!!