I currently own Sony A7RIV, A1, A7SIII. I previously owned Sony A6500, A7RII, A7RIII, A7III and A9.
I came to Sony after over four decades of using pro Nikon and Canon gear, both brands, and I have no regrets on making the move.
The Nikon Z9 is a worthy fullframe mirrorless flagship from Nikon. If you still have a host of great Nikon glass, then i recommend you go with the Z9.
I sold all my Nikon and Canon glass when I made the move to Sony for its leadership and pace on innovation in mirrorless gear.
I would still take the A1 which can shoot in raw for the 30fps in fullframe, while the Z9 can only do jpeg for 30fps in fullframe. Yes the Nikon Z9 can do even more fps in reduced megapixel jpegs, which is indeed a nice feature. This is much more useful than Canon's R3 doing 150fps for just 1/4 second (a real joke among pros, lol) .
Looking at the World's Top News Services, they have all moved in the last 20 months to fullframe mirrorless for their staff still photographers and staff videographers worldwide. They all cite the distinct advantages of mirrorless and the innovation of Sony and Nikon for their choices.
Most have moved to Sony, and that includes the Associated Press, all Gannett Media outlets, Canadian Press, and UK Press. Their staffers are mainly using Sony A1, Sony A9II, Sony A7SIII, and Sony FX3 and FX6 cameras.
One major news service, AFP (Agence France Press) helped in the development and testing of the Nikon Z9 and they are using the Z9.
Here are what they all said are their reasons for switching, and none of their reasons had anything to do with any special payouts from Sony or Nikon, as UHH conspiracy folks and Canon fans like to speculate, LOL.
https://alphauniverse.com/stories/why-the-associated-press-just-switched-to-sony/https://petapixel.com/2021/11/17/sony-is-now-the-exclusive-camera-provider-for-gannett-and-usa-today/https://www.dpreview.com/news/4545693607/the-uk-largest-news-agency-partners-with-sonyhttps://petapixel.com/2022/01/31/canadas-largest-news-organization-moves-exclusively-to-sony-cameras/#:~:text=Canada's%20Largest%20News%20Agency%20Moves%20Exclusively%20to%20Sony%20Cameras,-Jan%2031%2C%202022&text=The%20Canadian%20Press%2C%20the%20largest,provider%20for%20the%20media%20company.
https://petapixel.com/2022/06/09/how-pro-photographers-helped-make-the-z9-from-prototype-to-flagship/If I had never made the move to Sony back in January 2017, I could have been happy as a pro using the Nikon Z9 once they had filled out their Z lens lineup. But I am totally happy using my pro Sony gear including my A1 and have no desire or need to switch back to Nikon or any Canon offering.
Sony allows third-party lenses makers to make E-mount lenses and the E-mount is the most widely produced and used mirrorless lens mount in the world with nearly 200 E-mount lens choices from Sony and third-party lens makers like Sigma, Tamron, Zeiss, Samyang/Rokinon, Yongnuo, Tokina, Viltrox, Meike, and more coming each month. This is another distinct advantage for Sony A1 users. Nikon is now allowing Tamron to also make Z-mount lenses, a good move. As we know Canon has said no way to third-party RF lens makers. A dumb Canon move IMHO, that will ultimately benefit Nikon and Sony.
Cheers and best to you all, whichever one you would prefer to use.