It's really all about your use case and priorities. I had a Z8, sold it (long boring story), and within 2 weeks regretted it. It's the best camera I've ever used, hands down. Easy enough mistake to undo this coming year, and I won't lose
too much $ at its current sale price of $3395.
Battery life for me and how I shoot is not a big consideration, I'm used to carrying a few EN-EL15's around anyway. And the Z8's battery life was never a complaint for me whatsoever. Weight / size for me is also not a priority (but I understand why it can be for some).
I guess the best approach I can suggest is to figure out how important the vertical grip is for you. The grip and cost difference would be my 2 primary deciding factors. If you even
think you'll want the vertical grip for the Z8 - just stay with the Z9. Because the MB-N12 grip experience on the Z8 is terrible - whereas the integrated grip on the Z9 is flawless. The Z8 with the grip makes for a HUGE setup, and I hate the way it looks and feels on the body. Ok I shouldn't say "feels", because it worked perfectly well. I just couldn't look at it without shaking my head
Hope this helps in some way.