I think CX MPO should be the best programme if you fly primarily in Y. You can make Gold easily with this pattern, and once you have Gold you can use the seat guarantee feature CX gives to Gold members, which gives you the ability to guarantee a seat in the lowest mileage earning class 72 hours before departure. The Gold card also gives a better chance on CX and KA award waitlist. If you are a CX member, you can earn miles in most fare classes whether flying on CX or AA to the States, whereas if you are an AA member you can only earn miles on the more expansive fare classes on CX. However, if you fly J or F, AA could be a better option.