There is something about the map that is misleading. Even though Verizon has a larger 3G geographic coverage area, more people get better 3G service through ATT. This is due to the fact that ATT has more 3G towers in the larger populated areas.
For example, in my area, Verizon phones get a lot more 3G dead zones. ATT has many more towers in my area (They just put another tower exactly 1 mile from my house.) This means that each Verizon 3G tower gets more use per tower than ATT, thus more likely to overload and slowdown.
IOW, ATT focuses on putting more towers closer together in fewer highly populated areas, where Verizon focuses on spreading their towers out to cover a greater geographic area.
They have to do this due to the technology difference between CDMA and GSM technology. The cost of covering a rural area is significantly higher in rural areas for GSM than CDMA because the towers have to be closer together with GSM. However, even in metro areas, GSM will often be brought to it's knees with an increase of population density such as a stadium event or a trade show, even a traffic jam on the highway. This is the major issue with GSM technology. We see this every year at CES as well as here in town during an NFL game- CDMA users like Verizon and Sprint have no trouble while ATT and T-mobile users can't make calls. As an event is repeated, the companies AT&T and T-Mobile just bite the bullet and improve the density of those towers within the repeated density change locations. Meanwhile CDMA companies have the luxury of simply increasing the spread of their geography into those rural regions not previously covered.
There is no simple answer to this other than a broad statement that CDMA will be better if you travel around the nation while GSM will be lower cost for basic service due to lower licensing costs. If you don't travel much and GSM works well in your seat, stick with it.
Plus, there is no truth to having a CDMA based provider and not being able to use the phone in foreign countries. Verizon will supply a phone for you wherever you need to travel and in many cases the phone will have service ( CDMA ) in countries that "aren't listed as compatible. That happened to me in Grand Cayman and in the Bahamas.
Even though Verizon has a larger 3G geographic coverage area, more people get better 3G service through ATT.
Can you point us to some documentation on this? I've done a comparison test here with an iphone, and my XV6800 downloaded a large video file (78Mb) 40% faster than the iphone side by side. I don't even consider this location very good for Verizon EVDO. I've been in locations like Washington DC where EVDO Rev A was downloading at 2.4 Mbs. Here, I only get 1.3 Mbs