For me this would be an easy decision. CX F is light years above UA F.
Just consider the amount of space and privacy: In the 747 CX has 9 seats where UA has 14. In my view UA F is more like a good business class, but not even close to what most non-US carriers offer in F.
I would credit the miles to AA, not CX. AA still has a fairly good availability for premium class award tickets a few days before the flights.