I can fully understand your apprehension, the threads on high end 24, 35 or 50mm lenses are neverending and mostly end up, if they turn out civil, with supporters for the various lenses fairly equally spread out.
However, with the right experience, one should be able to deduce which lens would be the right one for you, guided by an in-depth comparison that touch on some essential features like color rendering, contrast and OOF rendering, illustrated with images, while a pure sharpness comparison would not help me at all when lenses of this level are concerned.
Buth this is based on image rendering alone, and especially with birding wildlife equipment, other factors come into play that are equally crucial, like portability and AF performance, and yes, due to the immense magnification, images are going to look more similar than is the case with e.g. large landscapes containing a wealth of (color) detail on 5K screens or printed large.
I fully agree with you, that both systems with the various lenses will give you everything you need and more. I am a few months away from a Sony 600GM purchase, and even though I expect Nikon to be ahead on contrast (which is usually very high) and arguably also on color neutrality (Sony tends to be a bit too rich and warm), this plays only a minor part in my decision. The price of the Nikon 600TC, the limited versatility of the 800PF, and the bulk and weight of the Z9 have been the real deciding points.