Legend states that if you chant "NEV4" three times in a row in front of a bathroom mirror in the dark, once you turn on the lights, the NEV4 ghost will appear on your booking.

Other than that, hello ! To those who are really curious about it,

KLM offers a strong, reliable and unique experience in Premium Comfort but the availability on the network is scarce, and while Air France offers a good experience too on many more routes, the seat type may vary depending on the aircraft going from good to not good.

On the KLM side: KLM operates 2 daily flights to JFK in the winter season, one at 1pm and one at 5pm + an extra one in the summer :

KL641, the lunch one.
Almost exclusively operated with a B787-9/10 and always offering their Premium Comfort cabin that is equipped with very confortable Collins Aerospace MiQ seats.
Note: On Sundays in January, February and March, this flight is operated with a 772 without Premium.

KL643, the afternoon one.
This flight is operated with a B787-9/10 offering Premium Comfort on Mondays, Tuesdays and Thursdays until the end of the year, and on Sundays too starting 2024, and daily starting April 2024.
This flight is more sensitive to aircraft change and often switches from a B787-9 to a 772. (and 772 with KL gives a higher chance of swap to 773.) And while, the 773 fleet is partially equipped with Premium, the 777-200 is not.

And during the summer season :
KL645, the middle one.
Not daily, and is to be operated with A330 for the 2024 high season. No Premium.

All KLM Premium seats are Collins Aerospace MiQ.

On the Air France side :

Air France operates up to six flights a day to JFK, and aircraft changes on the schedule or last minute aircraft swaps are very frequent on that route. So an AF flight number is not the strongest prediction factor of the onboard product you'll get to JFK. It does't really matter if you're on AF2, 10 or 12. It's all about which aircraft type you have on that specific day. AF4, 6 and 8 have a very high chance to be operated with the Collins Aerospace Icon seat in premium, due to P product onboard.

Air France operates up to five different Premium type of seats on that route !

If you're on an A350, you could be flying either on a Recaro PL3530 seat (if seat maps indicates rows 10 to 12) or a Collins Aerospace MiQ (if seat map indicates row 15 to 18) which are both good.
If you're on a 777, you could be flying either on a Collins Aerospace Icon (the worst Premium seat ever made), a Recaro PL3530 (if seat maps indicates row 21 to 26) or Safran Airgo FX seat (more rare on that route and will probably happen due to AC change / if seat maps indicates 5 to 8). (All other seat maps mean Icon seat).

Concerning the service protocole, there is almost no difference between Economy and Premium on Air France. And while you get to drink out of a glass and have more elaborated food, allegedly, you're still eating out of an economy tray with wooden cutlery. KLM might feel a bit more elevated, but the difference is still barely noticeable.

For that particular class of travel, I would tend to choose KLM over AF.

Fun facts: KLM was actually the first airline to fly from Europe to NYC, in 1946 and the flight number KL641 has been in use for more than sixty years ! So, like the old timetable said :