Actually, it doesn’t say much. Verizon has a bigger 3G network than ATT, but they have lots of holes in it and per my users sometimes it just plain doesn’t work - and there’s no real 2G fallback for VZ.
Meanwhile, while ATT’s 3G network is a lot smaller, they have damn near universal coverage on 2G - and the iPhone can downshift.
Quite often my users with iPhones are the only ones on site with usable signal, when they’re out on rural construction jobs.
Right you are... here is the true coverage you get with AT&T 3G plus 2G EDGE: