Previous posters have some valid points.

But it all depends on who you are going to fly the most. If you are going to fly CO more than NW, go to CO. Elite level rank differently on each airline.

CO:

CO Plat
NW Plat
CO Plat Companion
CO Gold
NW Gold
CO Gold Companion
CO Silver
NW Silver

on NW (please confirm if I mis-stated):

NW Plat
CO Plat
NW Gold
CO Gold
NW Silver
CO Silver
Plat COmpanions
Gold Companions

The only thing that NW has over CO is it's choice of partners. NW still has HP in it's fold and you can upgrade on AS. It also has better integration and reward travel on KL.

NW ELite Earning Partners:

KLM Royal Dutch Airlines
Continental Airlines
Copa Airlines
AirAlps Aviation
Alaska Airlines
Hawaiian Airlines
Malev Hungarian Airlines

NW Non-Elite Earning Partners:

Air China
American Eagle
America West Airlines
Big Sky Airlines
Cebu Pacific
Garuda
Gulfstream International Airlines
Japan Air System
Japan Airlines
Jet Airways
Kenya Airways
Malaysia Airlines
Midwest Express Airlines and Skyways Airlines
Pacific Island Aviation
Swisswings
Transavia

CO Elite Earning Partners:

Northwest Airlines
Air Europa
Alaska Airlines
Amtrak
British European flybe.
Copa Airlines
Hawaiian Airlines
Horizon Air
KLM Royal Dutch Airlines
Continental code-share flights operated by other airlines (Your ticket must indicate a Continental flight number in order for you to earn Elite miles.)

CO Non-Elite Earning Partners:

Aces
Alaska Airlines
British European flyBE.
Emirates
EVA Air
Qantas


- HF