What kind of plane is it on Delta? If it's a 767, the seats are definitely not as wide as the KLM/NW models. For example, Delta has 2x2x2 Biz in a 767 which has 2x3x2 in coach. Northwest has 2x2x2 Biz in a 330 which has 2x4x2 in coach, as the plane is wider so you get more room.

Last year, I took the reverse of your option: the 330 from DTW to AMS and the KLM 747 from AMS to ORD. I thought it was a great contrast though I preferred the KLM service to NW.

So, if I were you, I would pick the NW/KL combo because the Delta seats are going to be smaller on the 767 and the rerouting options are much better in the event of flight disruption.

Here's my review of the KLM and NW service on the routes in question:

http://www.flyertalk.com/forum/showthread.php?t=182299