How's the camera in comparison to your iPhone 6? Battery life? And how's overall speed and fluidity of the system in comparison? Those are my main 3 concerns.
There is no question, the camera is better on the iPhone 6. However the camera isn't bad on the OnePlus. It's just a little slower to focus, and pictures don't come out as often/reliably crystal clear like they almost almost do on the iPhone 6. I have to applaud Apple for their camera software. Their ability to process clear, clean, beautiful images whilst still allowing for quick snapping is unmatched. The OnePlus one's camera software is actually quote good, with a lot of very cool modes that all give good photos. It just isn't as quick and snappy as the iPhone. This is probably the only one area the OnePlus falls short of the iPhone 6.
Battery life is an easy win for the OnePlus. Keep in mind I'm comparing to the 6, and not the 6+. The OnePlus battery life just keeps on chugging. There's no other way to describe it. I don't really bother charging it midday any more. I regularly get 2+ hours of SOT at 60+% battery life. The iPhone 6 battery life was fine, but OnePlus trumps it easily.
Cyanogen is fabulous. It's stable, very fast, and the extra features they've added to stock Android really elevate Android to a whole new level. I can't stress how much I'm enjoying their version of Android. Every time I'm like "I wish Android would allow me to do this..." Cyanogen answers with that feature. It's really nifty. And it's not overbearing like I've seen of TouchWiz. It's all very intuitive, natural, obvious additional features; features which I don't get why Google themselves do not add.
And yes, the OS is very fluid. Everything is zippy. No OS is impervious to the occasional stall here and there, but it is very rare. In fact, I found my iPhone 6 with iOS 8 far more buggy, slower, and less stable. I've had plenty of apps crash on me on iOS 8, whereas I've yet to really experience any crash on my OnePlus.
Hope that helps!
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I now have the option of buying a good condition Moto X 2014 for $350, 16GB. Between the two, which would be a better option?
Aren't the Moto X's camera and battery life (two things you specifically asked about of the OnePlus) two sore points? That's what I've read.