I need a trip by the end of the year to requalify for AA EXP, and I figured I probably would want to do a quick turnaround trip HKG-Australia-HKG. I don't have a clear preference yet on which city to go, but am more likely to pick between SYD and MEL (but am still open to other suggestions).

Do you have any suggestions on which might be a better airport to do a quick turnaround between CX flights, e.g. from CX 105 to CX 104 ? For example, do I need to go through immigration to go from arrival to departure levels, how much time it would take, etc ?

I assume the inflight service should be very similar anyway, right ?

And, as an AA EXP (or BA Gold), is it more likely to score a upgrade from J to F on HKG-SYD v.v. or HKG-MEL v.v. ? On the dates I am looking at, the SYD and MEL flights are full in J, and some full in Y as well. I am thinking, HKG-SYD probably would have more CX Diamond's/Gold's ? HKG-MEL might be a (slightly) less popular route with less CX Diamond's/Gold's, and that might improve my chance to get an upgrade from J to F ? Of course, I understand that this is all pure luck and I shouldn't expect too much...but, if I don't really have a preference about the city, maybe I should just pick the route where I might have a slightly higher chance of getting an upgrade...

thanks a lot !!