There really is no one perfect answer, some places will have good signal with one, some with the other, some with both, and some places will have no signal no matter what you choose. I will say this about the topic though, in the summer of 2017 I did a 4,200 mile RV trip in a big loop from Louisiana to Yellowstone passing through parts of western Louisiana, Texas, Oklahoma panhandle, eastern Colorado, Nebraska, then a loop around Wyoming up to Yellowstone, back to Nebraska, clipping the corner of Missouri, and down throw western Arkansas. Along much of the trip outside of Yellowstone (which has almost no cell signal), I streamed music from my AT&T phone Sirius radio app over bluetooth to the motorhome stereo, the app (at the time) only had a small buffer (maybe 5 minutes) and along the drive excluding the National parks, in canyons, etc. I rarely lost signal long enough for the music to stop, and never for more than 15-20 miles.
I have the (no longer available) $20 per month AT&T Mobley unlimited plan, which works well for me, though if I had to buy something today, it would probably be the OTRMobile $60 per month pre-paid bring your own device plan which operates on the AT&T network, and seems to not have the throttling when you go over 22 GB per month that many other plans now have on the AT&T network. They also sell devices, though if you want to pinch pennies, you can save by buying a used / refurb Netgear 815S off ebay for $50 or so, which will in some cases out perform the refurb Velocity you can buy from OTRMobile for in effect $90 ($190 including $60 plan, and $10 SIM card with $30 activation).