The 6P is the best phone available today, the S7 will have better hardware but so will the 2016 Nexus which will follow the S7 by about 6 months. As everyone says the big advantage of a Nexus is that you get the latest software immediately, with Samsung the delay will be many months. The other software advantage is that Google supports Nexus phones much longer than Samsung supports their phones.
The S7 will have a Snapdragon 820 which has much faster single core performance than the 810 in the 6P however the question is will the difference be noticeable, I doubt it. Processor performance has been good enough for a long time. The performance of the 6P is as snappy as you would want it to be, but so was the N5 which had a processor that was 40% slower than the 6P, the Internet is the limiting factor not the CPU. The screen on the 6P is a 2K AMOLED, I don't think there will ever be a screen that's significantly better. The DPI of a 2K screen is beyond the limits of the human eye so there isn't any point to more pixels. It's also plenty bright, but I suppose you could make a brighter screen. Battery life on the 6P is decent but obviously there is room to improve on all phones and I doubt the S7 will be better than the 6P in this regard.
I only buy Nexus phones because they are the easiest to root if need be and they always have the latest software.