The thing with coverage is that it is highly situational. I have much better experience with Verizon where I live and the 15-20 miles away where my parents live. However, someone else might have the opposite and find AT&T better.
Just because one or the other says they have better coverage nationally doesn't matter unless you are going to be at every place in the country at some point. Otherwise all that matters is where you are and where you go most often.
And someone posted a link to the maps earlier, those are computer generated on what they "expect" to have coverage based on licensing and radio power. That of course is subject to change based on actual environmental conditions, doesn't take into consideration trees or changes made in the field, etc. It's no guarantee of coverage.