I can think of at least three rational reasons to prefer Nikon to Sony (as well as some to prefer Sony to Nikon).
Nikon>Sony:
Build quality, durability, sealing: Both Imaging-Resource and Roger Cicala (LensRentals) have looked closely at sealing and build, and both say that the Z-series is built like a D850, just as Nikon says it is. The A7r III and below are notorious for lesser weather resistance and build, and both Roger Cicala's teardowns and IR's "get it wet" test bear this out.
Neither seems to have tested an A9, so that might or might not be different. Some reviews say "it's built like an A7r III", while others say "this is different" - in both cases without getting inside, so who knows. The fact that the weight differs by only 16 grams between the A7r III and the A9 suggests that the builds might be very similar - but that 16 grams could all be gaskets...
Interface: In my eyes, Fuji and Nikon have the best user interfaces in the business, and Sony has the worst. This is, of course, entirely personal (for example, I struggle when someone hands me a Canon, but that's almost certainly because I haven't used very many of them). Just from reviews I read and comments on here, it seems like my opinion is relatively common - Nikons tend to be praised for their interface, Fujis are highly praised - except for a minority of reviewers who hate them - while Sonys are often criticized.
Sensor (right now, and only the Z7): The A7III and Z6 appear to be using exactly the same sensor, while the A7rIII and Z7 are using closely related sensors. The Nikon Sauce on the Z7 sensor seems to have some minor advantages, and one more significant one (ISO 64).
Sony>Nikon
Body lineup: Nikon offers a choice of a good general purpose full-frame body or the present King of the Pixel Monsters. Sony offers a general purpose FF body and a pixel monster plus a dedicated sports body, a video-centric body (albeit a bit dated right now - but they'll replace it soon), a bunch of less expensive prior-generation bodies and some (not always sensibly positioned) APS-C bodies. If you count Nikon F and Z together, they offer all the same choices as Sony except a video-centric body, but you can't use Z lenses on an F body.
Battery Life: The newer "big-battery" Sonys will outlast pretty much every other mirrorless on the market, although the older "small-battery" models are among the worst.
Take your Pick
System Depth (other than bodies): Nikon has an advantage in flashes, remotes and other things where the Z system takes F-mount accessories (although modern Sony compatibility is also pretty darn good). Any light that offers TTL at all, for example, will offer "modern Nikon" as a choice. Sony is often also available, but may not be as standard in rentals, and some exotica may come in Canon or Nikon only.
The most important system depth question is, of course, lenses - and either one could win, depending on how you count. If you count only native, non-adapted lenses, it's Sony by a mile (with a note that Nikon is rolling out lenses as fast as they can, and making generally sensible choices).
If you count native lenses plus fully functional adapted lenses with a fast, reliuable same-brand adapter, Nikon wins. The Sony FE system is pretty decent, but 60 years of Nikkors is a lot to choose from - from 6mm to 1200-1700mm, plus tilt-shift lenses and oddities like microscope adapters.
If you include lenses on (possibly slower or glitchier) third-party adapters, it's Sony again, although I suspect all the adapter manufacturers are working on Z-mount versions of their FE adapters (there's no reason why any of them wouldn't work).
Poll
55 (24.6%)
