Definitely take J if available. I tracked August flights for months to try and get my wife in JAL F after confirming a seat in AA F... For most days, a seat in F never opened up, especially on NRT originated flights. I did have success INSIDE the final 3-day window where online availability isn't posted. The day before she departed NRT, I called the EXP desk and they were able find inventory and switch her into JL F.

Was it worth the time I wasted to constantly check availability? Probably not... even with the famous NRT catering, my wife said JL F was a bit of a let-down after flying CX F the week before. FYI, she flew NRT-LAX...