I can't speak on their coverage, but as far as their "towers" I can. Most carriers in higher traffic areas share the same "tower". Look up and you will see generally 3-5 sets of antenna on a particularly high traffic area. One carrier may own that tower, or the "landlord" of that area may own it. Ownership of the tower really matters not. Spacing doesn't really matter either....that 5-10 feet up or down the tower doesn't matter all that much. The antenna face is adjusted accordingly. And any potential interference is handled through bandpass filters.

Hope this sheds a little light on the whole who owns what tower and it affecting coverage for a particular carrier.