I agree Verizon is the way to go. I have a smart phone on Sprint, they have unlimited data and pretty broad coverage. Sprint voice roams on Verizon and I never pay roaming fees. My hotspot is on Verizon because their 4G/3G data coverage is much more extensive than anybody else out there.
The only location I have been to where the ONLY usable signal is AT&T is the beach at Montauk. When we lived in NY and camped there a lot, I bought an AT&T prepaid phone so I would have service. The problem with the prepaid phone is that the minutes are only valid for a fixed period then expire...the longer you want to keep them the more they cost...and if you don't keep active minutes in your account the phone goes dark and you lose your number.
I haven't used the AT&T prepaid phone since we moved to Louisville, PM me if you want it.