For most of the US, Verizon is likely to provide a better chance of finding a cell tower than most of the other providers. If you stick to major highways & cities, you have more choices. Use the coverage maps of providers to get an idea of what they cover in both 3G & 4G.
I agree with Don - If you are looking for large volumes of data coverage, you can't beat Millenicom's Hotspot plan. You purchase a Verizon 4620 hotspot from them & get 20GB per month for $70.00. No contract & you can suspend service for a month or two. If you stop it longer, there is a $20.00 restart fee. They do not provide voice coverage.