Strange as it is to say on FT, you could also just not bother with the miles on this trip. I agree with others that for you, AsiaMiles aren't going to be terribly useful. The number you earn will only qualify for the shortest of redemption flights, if at all. I suppose if you think you might fly CX again in the next couple years, you might as well join AsiaMiles in the hope that you can augment your balance and redeem an award in the future.

But for your travel pattern, another OW program would probably be better. The AsiaMiles earn/burn ratio is pretty poor. And I'm a great believer that it's best to have status with the airline you fly the most, unless your individual circumstances warrant an exception (easier to earn status with an alliance partner than on your own carrier given your flying pattern, etc).