I ordered my 6P on day 2 and received it Nov 9 so if history is any guide you won't be able to get the 2016 Nexus for at least 6 months. Nothing is known about the 2016s Nexus(s), there are only rumors. One of the rumors is that they are going to shrink the screen to 5.5 from 5.7, I hate that idea if true, I've gotten acclimated to 5.7 and I don't want to take a step back. The processor will be upgraded, probably to the same 820 that's in the S7, that would be great but the 810 in the 6P is no slouch, I'm very happy with the performance of the 6P. There are no rumors about the camera, the one important feature missing from the 6Ps camera is optical image stabilization, the S7 has it, would be nice if they put in the the new Nexus. The one other important feature that the 6P is missing is wireless charging, the Nexus 5 had it and I miss it, the S7 has it, hope that they bring it back, I gladly go back to a plastic case to get wireless charging, I think the idea that aluminum == premium is silly. Another feature of the S7 is a slot for a FLASH card, I don't care about that at all, I have 64G in my 6P which is more than enough, if fact 32G that I had in the Galaxy Nexus and the Nexust 5 were more that enough. Bottom line although the S7s hardware is a little better it's not enough better to justify the extra $300 that it will cost you.
On the software side a Nexus is the hands down winner over a Samsung. A number of people on this forum who have both the S7 and the 6P say that Touchwiz negates all of the performance advantages of the 820. A Nexus runs a minimum of 6 months ahead of any Samsung phone, the 6P came with Marshmallow, the S6 is just getting it now. What's more the alpha for Android N has been available for the 6P for about a month, if you are feeling lucky you could install it today. Android N won't be available for the S7 until about this time next year. If you buy an S7 from Verizon you can probably tack on another few months because Verizon is very cautious about updates.