I was facing the same thing and went with NW. Here's why:

1. I fly internationally on SkyTeam carriers a lot. CO is downright stingy with EQMs compared to NW when crediting from other SkyTeam airlines -- its the difference between one elite level and the next for me year in and year out.

2. I like actually getting upgrades rather than merely the idea that one is possible. I get upgrades on NW -- on CO even Plats aren't getting too many if the CO board and anecdotal evidence is any indicator. It's a simple numbers game: NW has a much larger percentage of first class seat available to upgrade into domestically.

3. CO flies Barbie Jets / RJs all over. I'm rearely if ever stuck on a commuter shuttle plane on NW -- full size planes only, please.

4. It's getting harder to redeem miles on nearly every carrier, but CO is among the worst. DL has recently made it their business to eclipse CO in thsi regard, but people still call it NonePass for a reason.

Now in all fairness, CO has a much better overall product on the plane itself -- both in businessfirst and coach. The difference for me is that NW makes it easier for me to get/maintain my status, redeem miles for int'l J, and has the space for regular free upgrades so that instead of appreciating how nice CO is in coach, I get to appreciate how nice NW is in first. Nuff said.

peace,
~Ben~