I have flown on all types of Northwest DC-9's (10/30/40/50) and I have flown a few times on a 737-200 (Aloha) and here's my experience...
The 737 offers a larger seat and more room. But, if you sit behind the wing, it is very noisy (on the -200, on the -800 by Continental and ATA, I have noticed a much more smoother ride and less noise). Window seating on a classic 737 did not curve in as much as it does on a classic DC-9...more on that in a second... Yes, the 737 classic thrust reversers are a kick to watch too!
The DC-9 in the forward cabin is a lot less noisier, but having a window seat can be a royal pain. The cabin, a few feet below the window, curves in dramatically and if you are a large person, you have to cross your legs to sit comfortably. Plus, the seats are very small, probably smaller than found on Skymark Airlines in a high capacity config...fortunately for me, I had only to sit in mine for a little over an hour.
-80, however, as configured by Alaska, offers a very large seat, and the window seat lacks the dramatic inward curve. Very comfortable.