I think that NW is the larger user of the DC9, and got slammed recently in the WallStreet Journal for deferring maintenance (front page, maybe 4 months ago) in CONTRAST to TWA which wasn't doing the same.

What TW got rid of were the L1011s and the 747s. They now have DC9s (all series), MD80s, 727s, 757s and 767s. By ridding themselves of the big planes they took their average age down significantly (much less than NW)

From TPA they seem only to fly MD80s.