As a non-Delta pilot, another factor I would consider is marketability should the unimaginable happen and you are furloughed. A DC-9 type rating of any stripe will not help employment prospects, whereas a B737 and especially an A320 type allows you to 'write your ticket' on a worldwide basis. I know, 'hiring 50/month indefinitely', however I'm sure the guys in 2000 and 2001 thought the same thing...