The JL 009/010 flights between NRT and ORD have the shell seats in J, which are flat but not horizontal. The AA J seats are also flat but not horizontal in J, and IMO are far inferior to the JL seats. Both AA and JL have horizontal seats in F. The importance of flat vs horizontal could be a significant factor in your decision.

Otherwise, I think AA F is generally a little better than JL J, but for me it's not enough to pay for with $ or miles. If AA miles are important, you get more flying AA than JL.

JL J (D) is far, far better than AA J (D).