Live in a suburban/rural area, travel much and/or want to make sure you've got connectivity wherever you go? Verizon or AT&T are probably a better choice.
I live in a rural area, travel quite a bit (domestically and internationally), and need connectivity wherever I go, and I find Project Fi works better for me than Verizon (haven't tried AT&T). The combination of T-Mobile and Sprint's networks gives me roughly equal coverage to Verizon in the US (there are places I can't get coverage with my Verizon SIM and places I can't get coverage with my Fi SIM, in about equal proportions), and Fi's international coverage is great.
Disclosure: I work for Google (on Android), but that has nothing to do with my use of Fi, other than the fact that I always have a Nexus/Pixel, so I don't have to switch phones to switch carriers.