It all depends on your travel habits. If you travel to Hawaii and generally fly sterage as I do then Northwest wins hands down with more reward seating than CO and you may connect with Hawaiian Air for no extra mileage charge if you go on to Maui or Kona
whereas CO hits you for an extra 10,000 making you burn 45000 One Pass miles to NW 35,000. HoKeY upgrade policy on CO is a joke
whereas on NW is is possible to get an upgrade but no World Busines Class just plain domestic FC. For Domestic travel I would recomend America West with the most generous FF program going at only 20,000 miles for a free ticket year round.
Thats just my opinion.