Nexus 6P because it has a great stock Android experience, solid hardware, and amazing build quality! Oh, you can't forget about that gorgeous display either!
I would choose the Nexus 6P because I heard the camera is a bit better, and that the 5 still has some issues for some users. Also, I would choose the 6P because my LG G3 is garbage lately.
6P, because I would be coming from the whale of the N6, so I need something comparable in size, but not quite a behemoth. Reason 2 would be the camera. I've had a Nexus phone since the Galaxy Nexus, so I have been deprived of having an awesome camera experience for 4 years. Help a dude out!OO