Ah this is all great feedback! thanks dudes!
I thought I was gonna get a bunch of anti-9 inch propaganda, but your opinions are super helpful.
This is pretty much exactly the intended use case! I don’t have any real desire to get airborne.
I assume these two statements kinda relate to each other? I’m definitely going with the airs, so curious as to why you think they should only run 8 in Jack? Is it because the pkps become significantly harder to manoeuvre on 9s?
I assume these are the ones you are talking about? which ones do you use @Venom121212 ? Kinda not so much concerned about balance as I am about hard compound / longevity.
Given that I’m gonna have the undermount battery and also need a 10mm riser in the front already to compensate for the split angles in the deck, I think 9 inch is the clear winner. I am a little concerned about the weight, but when my only previous mtb experience is Madness, I think I’m probably gonna be ok with the weight on a 2wd build with a much smaller battery 
manoeuvrability is also a concern, but not a deal breaker, I think for what I will gain with the bigger tires, will be a fair trade off here.