since you are not likely to get status on either airline, it doesnt matter since DL,CO, & NWA are all together now in DLs Skyteam. Id fly the carrier with the lowest fare and put the miles into the same acct no matter which I flew. DL still doesnt require a Sat night stay over on its domestic tkts. So if NorthWest(NWA) has the best fare Id fly with them and give them my DL# to get my miles credited to.
The only thing youd lose is the 1000 mile booking bonus cause you purchased it on NWA.com and not Delta.com. You would need to have a NWA FF# and use it to purchase your tkt then either call up and have it changed to your say your DL# or do it when checking in or at the gate.