Have you ever even considered video when you compared the cameras...?!?!
The quality of stills is only half of the camera story. The other half is video, and it's here where the Nexus 6 camera truly shines. Try Chromecasting (or AirPlaying) an iPhone video to a 4K UHDTV and you'll see why: 8MP cameras don't pack enough pixels to shoot 4K video (the equivalent of an 8.3MP still). 13MP cameras, like the one in the Nexus 6, however, do indeed pack in enough pixels to record such videos. Also, it's only in low-light conditions that any difference in camera quality is noticed at all. AndroidHeadlines, if I'm not mistaken, actually did a side-by-side comparison of Nexus 6 photos and iPhone 6 Plus photos... with minimal, if any, difference in camera quality, the most minimal being in the case of those pics taken in bright sunlight (which we have plenty of here in SoCal IMHO).