AA and JL's trans-Pacific JV is starting, so I guess if you fly primarily on J and F AAdvantage would probably suit you better in this case. However, if you fly mostly in Y, I would still suggest MPO, as you will have a chance to use seat guarantee on CX when flights on AA & JL are sold out. If you book on AA's code, all fares except O (which I don't think is sold in Taiwan) earns 100% miles on MPO anyway. I guess it depends on your preferred class of travel and whether you think seat guarantee is valuable (if your plan changes often). I'm not too familiar about JMB, but it seems that they only give 100%miles on full-fare Y, so if you fly in Y I guess JMB is not for you.