When getting down to brass tacks, I'd say that for 99.99% of potential buyers, the decision of whether to buy a Z8 or Z9 comes down to ergonomics and initial cost. The Z8 has the benefit in initial cost. As for weight and how it feels in the hands with various lenses attached, I'd wager that for most the Z8 wins by a less considerable margin, but still wins. I know two local professionals who are using the Z system Nikons; neither is using the Z9. One is using Z7s and the other is using the Z7 and now also the Z8.
It makes me think that if the Z8 and Z9 were released at the same time, the Z8 would have outsold the Z9.
Either way you go, if you want a Nikon camera with great performance and great capabilities, you should be able to get what you want from one or the other, if not both of them.
And if you don't get what you want in a camera from either, then there are likely cameras from other manufacturers that fit the bill.
We live in a wonderful time to be photographers, at whatever level we practice.