As stated by BubbaX, KLM has a much more relaxed attitude towards moving around in the cabin and congregating in the back galley. Good opportunity to have an interesting talk with a fellow passenger during a long flight. They usually have a tray with snacks and drinks out in the galley for passengers to grab as well.
If you are flying on KLM metal, you have the option for a paid day-of-departure upgrade to the J cabin, either at check-in, in the lounge or at the gate, as long as there are seats remaining That is something that is not possible on DL metal. You can be offered those seats at online checkin, but it never hurts to ask at checkin or in the lounge either. Offers at the airport can be better than the ones at OLCI.
You are looking at premium econoy. Keep in mind that some AMS-US / US-AMS routes have quite good Flying Blue awartd pricing for a seat up front, and Flying Blue also has promo awards each month, discounting some locations. AMS-JFK or the other way around can be had for 57.5k miles + EUR 260 in business, which is quite a good deal IMHO. You could add on a Skymiles award ticket ATL-JFK on Delta and self-connect at JFK.
As mentioned by others, there is no real premium economy cabin on KL yet, but they have just announced a cabin makeover including actual premium economy, similar to Delta's product, starting this summer, probably by the end of July. There is another topic on that here. You can't buy this yet, but upgrades will be offered to customers ticketed on those flights as soon as they know what flights will offer the premium product.
One disadvantage for Flying Blue award tickets over Skymiles: the EUR 50 redeposit fee.