J on Star is invariably cheaper than J on Oneworld. Via PPT only NZ has a nice J, otherwise J on the Polynesian airlines (TN, FJ, etc.) is more like Premium Economy. I used to fly SFO/LAX-HKG-SYD/MEL routinely when CX had a nice F (and QF did not), it has lots of advantages. No jetlag, lots to see in HKG which I find a much more interesting city than SIN. However QF has the best J product at the moment on their A380s, but that operates only LAX-SYD and LAX-MEL, so you would need to fly AA SFO-LAX which does offset some of the benefit. Oneworld has a nice fare product called Circle Pacific (you must fly across both north and south pacific, so must go via NRT or HKG using CX/JL/AA and SYD-LAX on QF, with lots of variants on that and flexible routings).