The real difference comes down to space vs. miles tradeoff. Clearly you get more space in F, but food is exactly the same--take a look at the menus, precisely the same meals, albeit plated differently. Amenity kits are the same also. If miles are no object, go for F. I just did r/t to NRT in J and slept like a baby on both flights. Would F have been preferrable, sure--but J is perfectly fine if you want to conserve a bit on the miles. I'd go with the previous suggestion: J one way, F on the other to compare & contrast....