If most of your flying is on CO, it might make sense to get a CO account. With the likely approval of the DL-NW merger and possible affiliations between CO and (?)AA, you may not be able to credit CO flights to DL or NW much longer.

Likewise, DL doesn't give some elite privileges like domestic upgrades to CO or NW elites. I think you may be able to get elite seating (like exit rows) in coach, but if you're a CO elite and fly on CO, you might find yourself in F.

Finally, my understanding is that you can often get around the CO 50% EQM thing if you're a OnePass member and actually flying on CO, but this doesn't work with a NW or DL account. You need to book your flights through their website, so it may not work for business trips either, but you still get the opportunity for more EQMs if you're flying for pleasure.