I would recommend flying on AC and crediting the flights to Mileage Plus. You can use your Mileage Plus miles for redeeming on AC, NH, SQ, OZ, and UA for destinations all over Asia. I wouldn't recommend starting all over in a new program. Stick with Mileage Plus.

AC has more comfortable aircraft on their route compared to AA/AE. You'll appreciate the roomier feel of the E175 when sitting on the aircraft dealing with ATC or weather delays in/out of LGA.