There are many factors, which is why the decision on choosing a mobile network provider always comes down to what works best for your circumstances. Which bands is T-Mobile providing in your area vs AT&T? Perhaps there is a higher concentration of T-Mobile subscribers in your area. It's highly situation dependent.

For instance, we recently moved a few miles away, and it's a world of difference on T-Mobile. At the old place I couldn't pick up a 5G signal with my phone. I had to go down the street. At the new place, I can. I remember years ago I lived in an apartment building with concrete walls. The only way I'd get reception there is if I went out onto the balcony, and I had to rely on Wi-Fi calling for the time I lived there. I still stuck with T-Mobile because elsewhere it worked well for me.