i'd pick z750 simply because its lighter and has more horsepower.
I'd assume reliability isnt an issue since they still have loyal users in this country.
Parts availability though.. depends if you are servicing at a proper mechanic.
Some people that use z800 I know says parts are still not hard to get (gasket, valve, rectifier, etc.)
Maybe its the same for xj6 and z750.. best to ask your trusted workshop.

I don't think they are hot bike also since their equipment is pretty standard and old.