If you keep getting into arguments over which mobile carrier has better coverage in a particular area, you should consider dropping two bucks on the Coverage iPhone app. It's a rather basic app, but it should resolve the arguments.
Coverage will allow you to toggle back and forth between coverage maps for the four major US carriers as well as allow you to flip between viewing 3G, 2G, and roaming coverage for each. Pretty simple, but also rather handy. [iTunes via BGR]