I was on Verizon since the days they were named Bell Atlantic Mobile. They were always fine.
I switched to ATT and got an iPhone. ATT seems fine too.
I don't think that in most places, the choice makes any real difference. In areas of low population density, however, the choice of carrier is extremely important. When I go to Vermont, there are areas that are dead to Verizon, and others that are dead to ATT.
If I lived in NYC or SF, likely I'd say that Verizon is a better choice, but my guess is that some local company has all 4 national companies beat in the metro area.
This carrier lock-in, which Apple not only promotes, but profits from, hurts consumers. We have less choice of carrier than would be optimal.