Avanti, do you travel to the boonies? We have devices on all three networks and it seems rare for T-Mobile to have service where there is little population.
We certainly do. However, it is true that our sample of really far-out camping has been somewhat limited since we started the T-Mobile experiment. What data I do have is quite encouraging, though.
I stayed away from T-mobile for many years for the reason you mentioned. They had earned a reputation as a good "city" carrier. However, by all accounts they have been building out rural 5G coverage at a steady rate for a while now and it is starting to pay off. It has gotten to the point where they have started to brag about it:
https://www.t-mobile.com/coverage/small-towns-rural-areas
If you decide to review the Net's opinion, be sure to ignore anything more than a year or so old.
My guess is that Verizon still has an edge in rural coverage, but it is shrinking rapidly. And, where they do have service it is uniformly excellent. Plus T-mobile is SO much easier to deal with. Like, they answer their phones promptly with real humans. Mostly nice ones, too.
If you value maximum connectivity, you still certainly want two carriers. But, IMO it is no longer true that they should be AT&T and Verizon.
P.S. - For those who haven't tried T-Mobile, it is cheap and easy to give it a try. The secret $10 plan is really $10 (if you ask nicely, they will waive the activation fee), and there is no contract. Also, if you have an unlocked eSIM phone, they will let you try it out for 3 months for free:
https://www.t-mobile.com/offers/free-trial