Remember the blue vs red 3G coverage maps. At least with Verizon you'll be able to fall back to 3G outside of LTE. With ATT, there might not be any 3G to fall back on.
The thing most people dont realize about these coverage maps is that you won't get VZ's full speed 3G in most of that area. And when you do get full speed 3G, you are maxed at a theoretical 1.2mbps, with ream world of 700-800kbps. Outside major areas, you are going to get like 20-30kbps.
Vs. AT&T, whos 3G delivers 3-5mbps right now.
From what I understand it has to do with the wavelengths of CDMA vs GSM. You can think of CDMA as similar to AM radio, penetrates walls and goes long distances, but has limited capability/bandwidth. GSM is kinda like FM radio. Doesn't go through walls as well, or travel long distances, but the quality is much better when you have it.