A direct comparison of the OnePlus vs the N6... tough one in all honesty. I'd say if you took the networks out of the equation I'd get a OPO. It isn't missing any features in comparison, it has a GREAT battery, the OS is awesome, and the price CAN NOT be beat. If you're able to get one without any funny business I'd say get one. If the OPO worked on VZW I'd say it was a no brainer all the way.
I used the OPO on TMo for 6 months. All through last summer for work and vacationing around the U.S., and what I found is that TMo is trash compared to VZW. The network was the achilles heel of my OPO. If you live someplace that has a good signal AND you never travel anywhere then TMo is great, but if you find yourself in various cities or towns throughout the year you'll immediately see its shortcomings.
My N6 works on VZW, is vanilla Android, and allows root. That's EVERYTHING I want. So all things equal OPO wins , but the real deciding factor is the individual.