The Upgrade Travel Blog says you should avoid Northwest DC-9s because these aging planes are increasingly responsible for delays, with a single plane causing 2564 minutes of delays so far year to date.

I say you should avoid Northwest DC-9s because their first class cabins offer a tighter pitch (34 – 35 inches) than United’s economy plus on an Airbus A320 (36 inches). And the United Airbus offers channel 9 air traffic control, rather than no inflight entertainment at all.

Who needs unlimited domestic upgrades when that just means first class on a Northwest DC-9?