Assuming this is leisure traveling, flying KLM opens the possibility of a stopover in AMS. AMS-FRA is a very short flight so the fact that it's Europe Select wouldn't make that big of difference.
Here's an idea:
Fly NW on the way there and KLM on the way back. That way, you avoid the immigrations, customs and luggage re-check hassle in DTW, which I think adds a lot of stress to a trip. You wouldn't worry about that on your way there and another advantage would be that you'd get to come back and give us a comparison
(the latter would be the "priceless" part in a mastercard commercial)