As someone who has both flown JL from JFK-NRT in F, LAX-NRT in J, and on EY F LAX-AUH then LHR on J, I would go for EY.

Yes the F on JL is nice, and the cabin is great, the food is good, and the service is good. But I think you get all of those with EY too. EY's J is a really really great product. Another thing you are not factoring in here is that JL's policy is to keep their cabins at 76 degrees Fahrenheit. I found it impossible to sleep on my flight's because of the overwhelming temperature. I complained to the FA's three times, only to be told that other passengers were too cold to have them turn down the heat, and given bottles of water. After my flight I wrote a letter to JL, and it appears it is company policy to keep their cabins at 76 degrees Fahrenheit......?