For many years, I've carried both Verizon and AT&T phones and devices. Sometimes, its been two phones. The reason for this is simply that I travel far and wide, domestically and internationally, and there are often places where I will get better signal with one phone versus another. When that happens, I put the bad-performing phone on auto-forward to the good one. Once Google Voice and Ring Central became viable options for me, those are now my primary numbers and both phones are set to ring on call blast. Sometimes, my Verizon will ring, other times, the AT&T will ring. Occasionally, both will ring.
My biggest single beef with AT&T, and this behavior predates the iPhone by many years, is that even with a great signal and in major metro areas (NYC, Chicago, Dallas, LA) I get frequent call drops. I've got good friends who work inside of AT&T and they've all told me that this problem has little to do with their network coverage and a lot more to do with their software that is used to switch you from tower to tower as you move around. I've seldom (almost never) had a call drop when I'm stationary with AT&T, assuming I had enough signal to establish the call in the first place. But when moving, I'm lucky to go 15-20 minutes on a conference call without a disconnect.
With Verizon, I've never had a call drop, even while moving. And for voice, that's worth every penny to me.
My primary is now a Verizon iPhone. I still carry a cheap Android phone on AT&T and will until my contract runs out. But once it does, I think I am to the point now where dropping them altogether just makes sense.