I think either phone is going to have a large screen and a nice camera (the camera on the N6 is great).

I fully admit an extreme bias toward Nexus devices, but the OnePlus is highly regarded.

The only thing I dislike about the N6 is the size. It really is a behemoth. The phone/screen is larger than the Galaxy Note or the iPhone 6 Plus, and while it's gorgeous the battery takes a hit due to the high resolution. I can get through a day but just barely, and most days I end up charging it at some point (but it comes with a super charger so that helps).

The main difference between the phones is going to be Cyanogen. Several of the folks here really like it, but I'm not a fan. I've loaded it onto at least three phones and just didn't feel like it was stable, but I'd imagine a phone designed for it would be a little better in that department, and my impressions were made a few years ago so they've probably improved things a bit.