I recently considered changing from Verizon to AT&T, and I used the RootMetrics map to look at overall coverage and localized coverage in the places I go most often.
RootMetrics Coverage Map
The Verizon coverage at my house is spotty - most everywhere else I go, though it's fine. I chose to stay with with them because I'll eventually (soon?) be able to take advantage of wi-fi calling at home and not worry about the signal issues anymore.