JJ - i think the decision would very much depend on your long term thoughts.
The Sony & Nikon are both exceptional cameras; of that there can be no doubt!
At present some reviewers are saying the Z6 is better but in reality they are 'very close' and the body comes down to user preference really - rest assured the next generation from both manufacturers will be even closer and will come down to which name looks best on the camera!
We photographers know that the real investment is in the glass so this is really where you probably need to think before building a range of lenses up.
Nikon at present is behind in this respect but over time (like their F mount glass) they will have a very comprehensive range; I'd imagine that within 18 months it will already be good.
Nikon have introduced a new mount that their engineers promise all sorts and they have 'shunned' the Sony mount - that is debatable by many.
So really it will just be what system you are comfortable with in the end as the performance of either system will be within a 'gnats ******' of each other.
It doesn't help you in the slightest but my views - sorry