FWIW I saw a J flight on AA metal last week, LAX-HND, for 60k miles. I was surprised and I don't expect this to be a common thing to find, but they do exist.

But if paying with miles, I would go with JL. The food is the same (AA says that they designed the Japanese menu in conjunction with their partner JL, but I think they are just serving the JL business class meal bought from the same caterer, which is excellent) but the seat is slightly better on JL IMO and while the service on AA was good, JL would be better. The only reason I was on AA metal was that I got a cheap BE ticket upgraded with SWUs. While the experience was better than I expected, JL is still better IMO.

Also on JL from many gateways, you have the option of F for an extra 20k miles, which is well worth it IMO.