Hate to say but verizon has the US covered. Thats just the way it is. If you want some form of 3G everywhere then look to verizon. And I hate to say but the map in the verizon commercial for Verizon and ATT is correct. But both have equal coverage, just not in the highspeed data category. And then there is T-mobile. Their map isn't worth putting in a commercial. 3G for t-mobile is basically non existing except for MAJOR cities. Sprint is not too bad.
Take a look at the maps.

Verizon 3G in red

ATT 3G in dark blue

cricket 3G in dark pink

sprint 3G in orange

T-mobile 3G in green (pathetic)
These are supposedly updated 2010 maps. So who wants to go to T-mobile?

As you can see the coverage for ATT and Verizon is very comparable. It has also been reported that ATT's edge speed is sometimes faster than Verizon's 3G coverage. Now one more bad point about verizon and sprint. They are CDMA. No sim cards. Accounts are worthless if traveling abroad. So if you plan on visiting europe or asia you win with ATT.