Check your area on this FCC network map, and click on either AT&T or T-mobile (LTE Data and Voice) to see which carrier has better coverage for you.
Keep in mind that AT&T has a list of “approved” devices, so if the phone you’re thinking of using isn’t on it, then they won’t activate it. T-mobile is less restrictive. In any case, any phone you want to use on any U.S. network must be certified by that network for their own particular VoLTE implementation. Otherwise, they won’t allow it. Be aware of that if you’re bringing your own device that you yourself installed /e/OS on.
If you’re planning to use the Murena phone sold by e.foundation, then check the links that @tcecyk posted, to see whether others are using it on AT&T and/or T-mobile. (I haven’t looked, so I don’t know.)
Remember, too, that you can usually get cheaper plans with resellers (MVNOs: Mobile Virtual Network Operators) of T-mobile or AT&T (or Verizon, for that matter); they use the three major carriers for infrastructure, but have their own pricing, bundles, and options, usually at the same data speeds. (If you need help finding an MVNO, you can ask here for opinions and ratings.)