Depends on if it is a new JAL 787 or not... If it's the older JAL slopey seats (if the seat map shows the center section as 3 across, it's an old config), then AA is going to be better. Otherwise the new JAL J seat is flat and as good as AA.

Note that JAL's food/beverage is going to be better than AA, but JAL also likes to keep their cabins toasty warm which can be uncomfortable.