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There are so many variables that it's tough to make an intelligent recommendation without knowing more.

What class of service will you be flying on your paid tickets? If Economy, will you be on a deeply-discounted fare? (Some FFPs award less than full mileage when you fly a partner carrier on deeply-discounted fares.)

Will you want to redeem your earned miles for domestic flights? International flights? A combination of the two? And in what class of service?

What do you value most in an FFP? Free flights? Upgrades on paid flights? Lounge access? And how many total miles would you expect to fly in a 12-month period?

Given the fact that you may be flying to HKG with some frequency, you might want to consider joining a oneworld carrier's FFP, since Cathay Pacific serves HKG from LAX, LHR, and lots of other places. Both AA and BA fly LAX-LHR nonstop. (And LAX is an AA hub, which should be considered since you will be flying domestically, as well.)