I know it all has to do with where you live and where you travel, but looking at the data maps alone....AT&T wins it, from country to county zoom levels. Is AT&T just being optimistic? And, if AT&T covers more area and T-Mobile is faster (where they cover) forget Sprint (non-issue) what is Verizon's claim to fame?
I had figured covering the backwoods of Western Virginia would be dicey, as well as the deadlands of the Nevada territory, but even AT&T seems to have the more coverage there. I also know this isn't an exact science, but even then, unless someone colored outside the lines, the differences are more distinct.
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From a Western US point of view, I have Verizon personal and AT&T work. AT&T does seem to have a more generous map as there are more areas where I have Verizon signal and no AT&T signal even though the AT&T map says I should have a signal.
The question I seriously have is that coverage maps can claim anything they wish but in actual usage does it match up? I was looking at the map for AT&T 4G LTE and it basically shows pretty much all of the continental US covered. T-Mobile shows lots of gaps in coverage for the continental US. Sprint shows less gaps than T-Mobile but more gaps than AT&T. Verizon Wireless has more gaps in coverage than AT&T.
I also used the maps that were listed on each providers website so I wasn't using a map provided by anyone who could manipulate the data to skew the results in their favor. The one thing that I do know for sure is that Verizon Wireless is the only provider who had coverage on my college campus inside the basement of campus buildings. (was that an objective test? yes on the basis I tested all four carriers and only one passed).