kdinino is right, NWA has just more room and a little better meal. Last time I flew first class on NW it was a late night flight from DTW to TPA. Food was okay. The flight attendant forgot I had a scotch in my glass a before I could stop her she refilled my glass with white wine.

She was
sort of apologetic and got me a new scotch.
Anyway FC is not what it used to be. When I can get CO FC it is much better (my experience). Domestically I think it is the best
Delta is a downer to me because when I fly them I never get upgrades with CO elite status...so my vote goes to NW over DL