You essentially get the same benefits on each one. That said, CO will board their elites at a certain level before NW does theirs. Also, in general, you will getupgraded more flying NW in general. Were it me, I would bite the bullet and get the NW membership for a few reasons:

1. CO only gives you 100%EQM on its own flights if you book it through their website. This screws their business travelling members, CO knows about it and doesn't care. NW will let you get 100% EQM on all of their own flights.

2. There are many more mileage specials with NW

3. There are many more mileage run specials with NW

4. NW gives you the best chance at upgrades. At only a silver level you won't get upgraded much on CO, but you will have a better chance on NW as an NW silver.