This is all well and good but you get what you pay for.
Lemme put this out here first... I HATE VZW. I cannot stand what they do to devices. (skip to the last sentence if the rest is TL;DR)
I used a OnePlus One for the past six months on TMo while keeping my VZW MotoX as a backup. I traveled all summer long and always had both phones on me and often checked out both to see how TMo stacked up. I live in NoVA/D.C. and I traveled to Rehoboth Beach, Philly, NYC, upstate NY(Albany and Climax), New Orleans, and all through FL (from panhandle to Miami).
Straight up, TMo is half-steppin'. It's excessively spotty, and the majority of areas in between major metros are either uncovered or covered by REALLY bad 3G. If TMo is the top network in your area you are in the minority. Good for you, but it's not the norm. Outside of cities I found myself pulling out my VZW phone and having 2 bars of LTE when my TMo phone was at either barely holding on to 4G or at a constant 1-2 bars of 3G, which isn't good. It was most notedly apparent when driving in rural areas and the nav on my OnePlus would switch to dead reckoning navigation because there was no data for the maps. I would check the Moto X and have 2-3 bars of LTE. I'd tether the OnePlus and I'd be back in action. This is a big deal because the times you REALLY need your phone is when you're away from the comfort of your home area. Anyone can walk a block and get a good signal, but what about when the entire region doesn't have service... that's a BFD if an emergency pops up; THAT is when a cellphone earns its keep.
In large metros I found the service was "acceptable" and well covered with strong LTE, but even then it still had its moments. For instance, a month ago I was in the rain and parking my car at rush hour on K Street in D.C. (P.J. Clarke's). I got lucky and found a parking spot but I had to pay using a parking app. I tried to download it on my TMo phone... 1 bar of 3G... in the middle of D.C.! I walked up a block and down a block... nothing. Usually you can walk a block and you'll get a signal, but this time was a total bust. I went back to my car and pulled out the Moto X... 4 bars of LTE. I tethered my OnePlus (which was a reoccurring theme over 6 months) and downloaded the app and paid for parking. I'm not saying VZW was perfect, but there's a noticeable difference in the block-by-block consistency of service in large cities... very noticeable. And this isn't just in my home city, it was the case everywhere I went.
The other thing I noticed... Tower saturation. At large sporting events or anytime large crowds of people gathered my TMo phone just couldn't cut it 3 out of 11 events (yes, I noted it). It would show full reception but the data speeds were abysmal. I specifically remember my VZW phone being a complete DOG during baseball games but this issue has actually disappeared in the past two years... with TMo it was a current issue.
So let's say, 80% of the time pretty much any mobile carrier will work at an acceptable level for any given person if you live in a large city. I acknowledge that outside of the city it totally depends on who has a tower near you. But, in my own personal head to head test VZW came out a big winner in these situations: network consistency, traveling THROUGH rural areas, and handling network saturation. VZW had a usable (mostly LTE) signal time-after time-after time, in town-after town-after town, on block-after block-after block. It's simply a better network, period. YMMV, but I traveled quite a bit by land over the past 6 months, and you won't convince me otherwise.
I'll also say I would still be using the OnePlus on TMo if VZW didn't allow the Nexus 6. But I'd definitely have my VZW phone as a backup.
Right now, using my N6 on VZW leaves me EXTREMELY satisfied in my cellular situation. I HATE VERIZON AND THEIR MEDDLING WAYS.
...But dat network.
