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I believe that AS generally has better redemption rates, but mixed flexibility and lower redemption availability (i.e. sometimes, they offer 1 seat less on CX awards versus AA)
With AA, you can get a little bit more flexibility (e.g. possible to book SYD-xHKG-TYO as one award, whereas AS asks you to book them as two).
But if you want to do SYD-HKG-JFK say, the value is incredible (at 60k AS points) AND you get a free stopover in HKG (even on a one way ticket!)
So, it really depends on what routing you wish to do... and we can better advise if we have more details...