AF J is better than KL J in all fronts, in some the difference is marginal, in others it's bigger, but everything is at least slightly better, the food, the drink (wonder what champagne they offer nowadays), especially the lounges. With recent state of AMS not even transfer experience is better for KLM.

edit: unless you lose the lottery and end up on COI plane. I will grant that that is worth switching to KL.