The bonus miles are AsiaMiles only, not MPO. So if you're worried about status (doesn't sound like that's entering into your calculations), only the 10% COS bonus is relevant, not the double AM offer.

Really, only you can answer this question. I'd decide first based on comfort vs. what you would do with that extra ~HKD6000 on the ground. If AM enter into it, I'd think about how quickly you tend to accrue miles through your regular flying. If the bonus will make the difference for banking enough to redeem a reward and it'll be tough for you to earn the extra miles otherwise, it might be worth it.