I suppose it somewhat depends on where you live and travel. For us in Oregon traveling in OR, CA, WA,ID, MT, AZ, utah etc. Verizon has better coverage than ATT. our truck has ATT, as do many friends. Our cell phones are Verizon. We find more places where ATT doesn't have service while verizon does than the other way around. Having both is nice for the occasional condition when Verizon has no service but ATT does. What is disapointing is that there are major streches of interstate highway we have traveled on where Verizon has service but ATT has no service for stretches of 50 miles or more.
We have quite a few remote campsites where cell service w/o a booster is non existent. With a booster again Verizon wins over ATT. Friends with ATT quite often don't get reception with our booster but we get a strong signal with Verizon.
Our daughter has sprint and it comes in a distant third. Even at our house sprint reception is very weak.