Originally Posted by grizzly
*snip* If you're going to be flying mostly low fares on NW (ie 50% EQM if you credit them to OnePass) I think you should switch since you're going to have more miles with NW. If, on the otherhand, your NW flights are going to be mostly from the higher fare classes, it probably makes sense to stick with OnePass so that all of your CO fares get 100% EQM. If you're flying low fare classes in both, I'd still suggest just picking whichever one you'll be flying more and going with that. Personally, I often fly CO more than NW but still stick with the WorldPerks because I think it has some substantial advantages over OnePass.
Geography should play a factor in your decision. Stick with the hometown hub advantage unless you're flying frequently into a hub (for example, if you live in Chicago, then stick with NW for frequent connections in DTW or MSP... unless you're frequently flying into CLE or EWR).
I agree with grizzly. WP has better reward partners in SE Asia (GA and MH) and better presence in northern Asia and trans-Pacific routes.