How much could you sell it for?
It's a huge screen difference. The Nexus has better software but the S6 has better hardware. I recommend the Nexus 6p since it will have more support for years to come. Exynos is amazing as a 14nm tech marvel, but the boot loader and Knox is a pain for devs. I love the S6 hardware, but the software is buggy, gets bogged down easily, and doesn't help the fact the battery is too small.
Time to move on, get another AMOLED screen, better software, extreme carrier flexibility, dual front facing speakers, etc.
The S6 is still holding its value quite well here in Vancouver Canada, looking at online listings shows people selling them for $500-$600 which would essentially make the 6P only cost me an additional $250.
The S6 is a hardware beast, the camera is simply exceptional and the display is a sight to behold while the processor still dominates benchmarks. My issues stem from the lackluster battery life and poor software, I charge my S6 multiple times a day and deal with random software glitches on a daily basis
The latest is a bug that came with the 5.1.1 update for Canadian S6 devices, if you have LTE enabled, all your incoming phone calls go to voicemail and text messages are delayed up to 30 minutes. The only fix at this time is to disable LTE and use 3G while some are forced to resort to 2G, this bug has been there for well over a month and a half now and there doesn't appear to be an ETA for a fix.
The Nexus 6P worries me though, I have concerns regarding the camera and I have concerns regarding the battery life.
I consider myself to be an avid photographer and regularly take photos throughout the day, I have become used to having a smartphone camera which is almost guaranteed to produce a great shot. I fear the Nexus will lack this consistency as Google lacks the pedigree in image processing and camera software.
The battery life in theory seems to be a non issue with the massive capacity battery but initial reports seem to present a different result, I'm eagerly awaiting the user accounts of battery life as the reviews have been vastly different in terms of battery life.
The reason I ventured away from the Nexus devices was solely due to Google's inferior hardware in the Nexus devices, both battery life and camera quality was inferior to competing devices and while the software was superior, I wasn't ready to give up the rest of the hardware for it. I feel this may have changed with the 6P though.
I'm still waiting for demo phones to reach stores so I can get a proper hands on with the 6P, until then I'll keep using the S6 and keep waiting on reviews.