I will call CO first to check the upgrade availability. If there is seat open, book it right away.
IMHO, CO's BusinessFirst beats Northwest First. It's worth the $200.00 difference.
Northwest First to Hawaii is basically their regular domestic First. 38" seat plus tray meal service. No in flight movies.
CO uses their BF plane to go to Hawaii. Often you get half lobster as appetizer, print menu with 4-5 selection. 55" seat with individual entertainment system. CO also allows you use their airport lounge, while NW won't.
The only problem is to get a CO seat. CO BF seat is hard to get. Make sure when you book CO, you need to get a H, K or Y ticket in order to get upgrade. If you can’t get seats from DCA-IAH/EWR-HNL, try DCA- IAH/EWR/CLE-LAX-HNL. I find LAX- HNL is easier to get, but the flight is shorter.
Good Luck!