Z6II vs D850? Only thing that matters is AF

lenses to be used are 105mm 1.4e and 500pf. shooting scenarios are low light performances and events with the 105 and hummingbirds for the 500pf. Used to have a z6 but it crapped out. local shop has the z6ii and d850 for about 900 each with 1 year replacement warranty. I know the better choice would be a z5ii but that’s not on the table right now. is the d850 tracking way better than a z6ii? thank you.