Verizon Wireless (115 million);
AT&T Mobility (107 million)
Sprint Nextel (55.6 million);
T-Mobile USA (33.3 million);
America Movil (TracFone Wireless) (23.23 million US);
MetroPCS (Merged with T-Mobile) (8.9 million);
U.S. Cellular (5.8 million)
Leap Wireless (Cricket Wireless) (5.3 million)
You also get better coverage for the most part with Verizon..
That being said, I love my T-Mobile / but T-Mobile is also pretty good around Alabama and the Birmingham area.