I have just completed a 7,500 mile trip around the US on my motorcycle (BMW K1200LTC). I went through New Jersey, Pennsylvania, Virginia, West Virginia, Ohio, Indiana, Illinois, Missouri, Kansas, Iowa, Nebraska, South Dakota, Tennessee, Wyoming, Utah, Nevada, California and a state or two that I've probably missed. The trip took 5 weeks.
On the road I carried my Verizon iPhone and an AT&T iPad. I also took along my MacBook Air. As most of the motels' WiFi wasn't good enough for streaming video, I spent a lot of time trying to stream over the iPad or tether to the iPhone. What I found out has convinced me that the next iPad I buy will be a Verizon one and I'll never get another AT&T smartphone again.
Simply put, I was in a lot of unpopulated and out of the way places. Only once in the entire 5 weeks I was gone did I loose Verizon 3G service. However, the AT&T iPad was pretty much useless for streaming unless I tethered it to the Verizon iPhone. Once I got out of major population areas the only coverage I generally got on the iPad was Edge, and often not even that. It was not unusual for the iPad to get no coverage at all. In major cities it was OK, but not otherwise.
Despite the maps you see on the AT&T website, 7,500 miles of travelling around the country clearly showed the superiority of the Verizon network. Luckily I didn't need any emergency assistance, but when I was on the back roads of Nebraska, Wyoming or Nevada the Verizon phone always continued to get a signal, and the AT&T iPad reported no reception at all.