KL use 747-300 aircraft which are not brand new but relatively new. They also "look" good both in and out.

NW use 747-400 aircraft which are sometimes quite nice but other times they are among the older versions of the plane. They also use very old DC 10 aircraft on certain flights.

Besides the aircraft the servicxe on KL will usually be better even though both airlines claim that the "World Business Class" product is equal.

Try KL and you should be happy. Nothing spectacular but nice.