In my limited experience, (I only flown them on the 787 and the new "Sky Wider" 773), they're comparable, with a slight nod towards JL for having more "good" extra-space seats on their 777s than CX. Especially the bulkhead seats have more legroom, and you have a guaranteed empty middle seat in rows 53-55 ABC/HJK on their 773s - pretty much the only plane where I prefer to sit way back .

Seat comfort is pretty much the same, as is IFE. Both airlines have yet to follow the industry trend of cramped 10 seats across a 777, and JL uses the same 3-3-3 economy layout as CX does.

If you have never flown on a 787 and JL operates one on your route, go for it. The experience even in economy is definitely nice (they have 2-4-2 seating), and you get the quietness, the better cabin air and the electronic window blinds as a bonus.

Noteworthy difference:
JAL cabin temperatures tend to be warmer than CX by a few degrees. That's something you either like or hate....