The key factor.

When I switched from CX to AA after the MPC programme was gutted for PEY/Y flyers like myself I did some research. My travelling is mainly TPAC, some Asia regional and HK-UK. Please bear in mind below is for PEY/Y passengers.

1. For each programme AA/CX/JL if in economy then flying on the others programmes are not going to be great for earning award miles, or mileage towards status.
2. If PEY and above then CX and JL on non-AA codeshares are going to be very good for AA status and miles, but domestic flying (which is not a major item here) means no lounge access.
3. For CX 6 return flights in lowest class business will get you to OWE on tier points, same in PEY & Highest price buckets Economy will get you to OWS, best not talk about lowest tier economy. But this is on CX only, AA & JL earn lower tier points.
4. But CX for these price buckets tends to be more expensive than other airlines (well into and out of HKG), so there is the CX premium dollar cost to be factored in.
5. JL is really geared towards Japanese and people based in Japan. Remember they have the JV with AA across the Pacific so the codeshare and JV comes into play.
6. For JL their are bonus points towards status which earn the same on JL as AA, lower on BA, and I believe not at all on CX.
7. For AA in economy should have no problem getting to OWS, and a mixture of economy and premium (including PEY) should get OWE.
8. AA at OWE will also help get upgrade certificates as well.

Suggest do some research, but if you are looking to re-locate to Japan, then JL is worth looking at, again if re-locating to HK, then CX is worth looking at, if not then longer term AA could well be best. Personal observation and others will give different perspectives.

Good Luck!

Nicc