I googled for a while and gave up, surely someone could come up with current stats. The only one I could find was from about 4 years ago. I am sure the A330s are nice but NW still has a lot of old DC-9s. And, NO, I am not bashing the DC-9...I don't mind flying on them at all.

http://news.minnesota.publicradio.or.../02/21_ap_dc9/