To set the record straight, thanks my growing annoyance with all those tit for tat commercials:
What Verizon shows as AT&T's lacking high speed 3G coverage is accurate. The really sparse map Verizon shows is not AT&T wireless coverage (nor does Verizon claim so), just the areas in which AT&T actually offers 3G data speeds. AT&T's claim that they cover 97% of the US population is also accurate -- they don't cover 97% of the US land mass, just 97% of the population living in the US -- and with no guarantees about data speeds.
The argument as to which one has better 3G is clearly impossible to settle. Fundamentally AT&T has higher possible speeds, but Verizon has better coverage.