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You don't need something better than the iPhone camera though. It might be as old as hats, but if it ain't broke...
Professional journalists, photographers and instadouches are all using iPhones and they take great pictures (in the right hands).
Your iPhone 6P is the second best iPhone (if you consider it second behind the 6S) and I'm not sure an 'upgrade' to a Nexus would feel that much like an upgrade. Just stick with the 6P until you actually need to replace it.
Fox-
That's why I got the iPhone in the first places, it work really well and the software combine make it easier to take shot, at the time there weren't many android phone that deliver similar result despite a superior camera.
I got till late 2016 to replaced it anyway, so I'm in no rush, I already take some awesome shot with it, but already pushing it to what it can do (low light, long exposure, limited to how much I can do in post-processing).
edscoble
While the iPhone camera as old as hats, the software are excellent which is why I'm using it for photography (much easier to go touring).
However right now the cameras on android phone are improving in an accelerated rate overtaking the iPhone already, so my choice is now the Nexus (big sensor over resolution, USB-C) or the LG (RAW and appear to have better software + SD card).