Go with AA... though if you mainly fly on CX and fly in economy, Marco Polo/Asia Miles might be better as low fare classes on CX/KA do not earn ANY miles with AA. But if you mainly fly in full fare Y or business class tickets, either option is okay. I'd prefer AA since 100% bonus is given for CX flights after you reach PLAT. Also, earn-burn ratio is much better with AA. Lastly, I've been treated very well as AA PLAT or EXP (or even as LAN Comodoro) on CX. Between 2009 and 2010, I flew about 5 segments with CX (and 2 as part of DONE4) and I have yet to see the economy cabin in CX! Three op-ups (ICN-HKG (B-fare), MAA-HKG (L-fare), HKG-ICN (L-fare)).