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The A9 almost certainly has the best AF in the mirrorless world, which I acknowledged in listing body diversity as a key Sony advantage. We haven't seen the Nikon body that's supposed to have pro sports grade AF yet. A7III to Z6 or A7rIII to Z7 is a pretty fair fight as of Firmware 2.0 (maybe a Sony advantage, but not huge). Throw in the A9, and the advantage is more distinct - but Nikon doesn't yet compete directly with the A9.
While I think there is a bit of a Sony advantage in the AF performance area, I find that there is a much more-significant stumble on Nikon's parrt in AF usablilty—but only in the continuous tracking/AFC part of the AF feature set. It's just more difficult than it should be to get the camera to lock onto the subject you want it to. Part of that is the decision not to have any visible focus confirmation in AFC, so you don't know when the camera thinks it has acquired focus, and part of it is the button presses required to move between trackable targets. It works, but its more dificult than I'd like it to be. So in this very small subset of its AF functions, the camera feels very much like a first generation effort. This is odd, and disapointing from the manufacturer who brought us the D5.
On the ther hand, in all of their AFS modes, my Z6 and Z7 out-perform my A7R3 because the camera gets there faster. Using AFS, the Sony always feels sluggish, like I have to wait for focus before the shutter will trip. The Nikons just feel snapppier. and for my work, that's more important more often.
For video AF, the Zs are as good as the Sony in every way, and give more control options that let me fine-tune the
speed of focus to match the shot. I also feel that when there is no face to focus on (where both cameras do very well) the Nikons make the right choice in
what to focus on more consistently than the Sony.