Pick the carrier you're going to fly and join theirs. CO gets a bad rep for redemption, and redeeming miles on their metal can be difficult at times, but they have availability on their partners as well that is usually pretty good, and that's what you'd be booking anyways if you were a flier on the partner.

There are a lot of 50% EQM earning rules amongst DL/NW/CO. I believe that all three now give 50% on the others' discounted fares (maybe not DL/NW to each other), but that will affect your earn rate.

I'm in CO and I like it. Of course, I also put up with EWR, so maybe I'm not a good barometer