If I were flying domestically in this situation I'd go with DL, internationally I'd go with AA. My reasons are almost entirely metal based as I just refuse to fly on the MD80. I know DL still have DC9s but they are reasonably easy to avoid compared with the plethora of lawn darts that AA insist on running.

It may be somewhat irrational but they just don't seem very safe these days. Although the 737NGs reputation has taken a beating over the last few months, I'd still trust my life to Uncle Boeing than the late Great Grandfather McDonnell-Douglass.

And I'm far from a nervous flyer.