I've had both. I had a little better luck in town and on the interstate to Billings with ATT. Honestly, they are both fairly terrible once you get out of town 10 miles.
Been with Verizon for the past 8 years travel a lot ... texas, florida, tennessee, nebraska, Dakotas and even in LaCrosse Kansas (look That town up lol) haven't had issues that I can remember.