I had AT&T for the last 9 years, but switched to T-Mobile when they started the military pricing. My service as been the same or better so far (Jacksonville, FL to Dallas, TX). The entire drive from Jax to Dallas, we only lost service for about a 2 or 3 miles stretch of road on two occasions, but that was the same when we had AT&T.
The only place where our service was slightly worse was in Emory, TX. With AT&T, we would get 1 bar in some areas of my parents land, but mostly no service. With T-Mobile, it was no service the entire time until we got in closer to highway 19.
My wife streamed directv now while we drove back roads from Emory to Commerce and back, and never lost signal (once we got away from the dead zone around my parents house).
As for price, we paid $233/month with AT&T for 4 lines.
With T-Mobile, we pay $133/month for 5 lines and 2 iwatch plans.