Platinum here. I mainly fly J. Have flown both AF and KLM extensively transatlantic past few months.

AF= seats are better in J than KLM. Much better.
AF = better food in J and economy.
AF = good service in J. Average service in economy.
KLM = good service in J. Better than average service in economy.

AF & KLM: Adding your DL # online for both airlines online can sometimes be very glitchy. Many times I had to provide them with my # at check-in or I had to call them in.