You asked
Nikon z6 vs sony a9?
Both are full-frame mirrorless cameras with in-body stabilisation, so the choice comes down to what you shoot and which lens mount you want to build on.
The Sony a9 was made for fast action, and its stacked sensor is part of that. Used, it's a bargain for sports and wildlife. For reach, try the Sigma 150-600mm f/5-6.3 DG DN OS Sports or the Sony FE 70-200mm f/2.8 GM OSS II.
The Nikon Z6 is the more general choice, a capable used body for stills and video. Its Nikon Z 24-70mm f/2.8 S II and Nikon Z 70-200mm f/2.8 VR S II make a solid everyday pair.
My pick: the a9 if your subjects move, and the Z6 for everything else.
Written by Claude from this site's pages.