It sucks that you're paying for F and riding in J for one transpac segment. Personally, that would be a no go for me, but I know how tough these awards are to get. Since you are doing an all-partner award, you can change your routing for $100, if Z comes available on the return. Keep checking!
Did you check TN as well?
Have you considered making it a Oneworld award (a premium of 35,000 miles per ticket)? That opens up returning through HKG on CX. I've found that QF has several dates with Z availability on its MEL-HKG flight (and vice versa).
As for your question, go with the upper deck if you've never experienced it before.