Simple answer is there will never be a system that stays ahead for long - they continually swap places and both have their strengths and weaknesses.
Canon is solid and in the right hands will easily produce award winning images.
That said, at this moment in time, Nikon has advantages in both ultimate sensor IQ and AF ability. Again - this is not to say Canon is bad in any way, just that Nikon has the edge at the moment. Actually, I'd say Canon has the edge in high ISO, but again this depends on your ability both in the field and at the computer in PP. This could all change over the next few years - a couple of years ago Canon lead the way for AF.
Bare in mind that what you need depends a lot on both the conditions you intend to shoot in, and your intended final usage, and you may find you can use something else completely - Andy Rouse knows a thing or 2 about wildlife photography, and last time I looked he was shooting Olympus gear after years of both Canon and Nikon. His pictures have continued to excellent.
The D850 is, to be fair, an awesome camera - if you think that 500mm is enough reach for you then the files will knock spots off your 7d2. The 200-500 will take a 1.4x as well, but when I tried it I found the AF took a larger hit than other cameras when a 1.4 is added. It should remain superb for fairly static subjects.
Get out in reasonable light, use good PP techniques and its surprising how little difference you will see in the end result between the systems - its not until you really push them that the respective abilities become apparent
Mike