If you are generally in J class to start with then in my opinion is AAdvantage is hard to beat. The biggest drawback to MPC is that there are no tier bonus'. While they treat their Diamonds very well, they also treat OW elites like their own. As a sapphire, you will be able to use any J lounge regardless of class, and in my experience if you do check luggage it will be tagged with business class priority tags even if you are in economy (as an EXP mine get tagged with First priority tags without me ever asking). CX is one of the few carriers I fly where as christep mentioned priority baggage generally works. With your AA plat status, you will still get your 100% redeamable bonus in addition to any COS bonus. On MPC you only get your COS bonus.

Now the drawback of course is that if you want to catch a cheap economy flight somewhere, you cannot earn AA miles (some do not even earn Asiamiles). What I did was join Asiamiles and also get the CX Visa card. I have racked up a ton of Asiamiles this way mostly through purchases, but also the occasional cheapie weekend flight on CX or for use on Dragonair flights to China. Asiamiles also has dining promotions and other earning opportunities around town.

Asiamiles are good to have as they are generally worth more than AAdvantage miles. It is also a good back up program for OW as you can use them for upgrades on BA (in addition to updgrades on CX), and should you want to fly BA transatlantically, then you can credit to Asiamiles.

Flying lots of CX business sectors particularly if you fly back to the US a few times a year is also an easy way to earn EXP (remember you in theory would still need 4 AA sectors per year to maintain your AA status).

In short the only downside I see to staying with AAdvantage is the lack of additional award availability given to MPC elites and the guaranteed Y seat. (that said I have always cleared Y flights a have waitlisted on quickly)