LOL, that's the million dollar question
Nikon has a new 600mm on the roadmap that I'm intrigued with. I generally like their ergonomics better, although I think Sony has them in a few areas when taken objectively. At the moment, I really like the Sony 600mm though - it's seems like the best 600mm I've ever used. It also seems to take the 1.4TC better than Nikon's 600 does. (I'm thinking about a formal had-to-head test). So, it's not just the bodies, it's the lenses too.
The big question is, what will the Z9 be like? I was out just this evening with the Z6ii trying the tracking mode on seagulls against a generally blue sky - it failed spectacularly. (Thankfully, the normal AF modes did really well.) The equivalent Sony tracking gets on target and sticks like crazy. Nikon needs to get to that level if it's going to stay my primary camera system. I really do want Nikon to succeed here, but when you shoot the two systems side by side for action, the differences in performance are obvious. Still, I feel like the Z6/7 are more of a mirrorless D750 and really aren't meant to perform like the a9ii (or even the A7R4). So, I'm cautiously optimistic.