I would find it difficult to decide, as well.
I was loyal to NW throughout the 1990's. Then switched to CO last year for better equipement, better food, etc. But my fleight originations and destinations changed somewhat, when I moved from Palm Springs area to San Diego. There is more Co service out of San Diego.

Out of Detroit you have MORE NW flights to choose from, and NO hope to Cleveland. Fortunately, you can use the CO OnePass for flight credits.

It seems now, that it is easier for me to get flights on NW.