I've taken both, albeit it's been a few years since I've been in WBC on KLM. In short, KL will probably have less hit-or-miss service than DL (and you'll get one of the Delft houses, to your comment), food will be a toss-up (read: not great on either of them), IFE will probably comparable (with a slight nod to DL, given it's a US airline), and the J class seats will be similar enough. Regardless of if you're on DL or KL, you have access to the same KL lounge in AMS, check-in, etc.

I personally prefer DL just because it's "easier" (if things go sideways...it can be a hassle dealing with KL policies vs. sticking with the known quantity that is DL), but all-in, I'd pick whatever works better for your schedule. One isn't that much of a standout vs. the other (unless you really want a Delft house).