I had been asked to reply to this thread and had dragged my feet--I think it's basically a wash between the two; it really depends on where you need to go and when. My wife traveled nearly constantly between BNA and LAX for about for years, and took the nonstop Sunday evening AA1307 on a regular basis, coming back either on a Thursday redeye through ORD or DFW or on a Friday's nonstop AA1974 from LAX-BNA (WN and AA are the only two carriers to offer nonstops between the two). She made EXP fairly quickly and was upgraded nearly every time. I took DL between Nashville and Seattle for a while and was generally happy with DL, although I found getting upgrades very difficult with DL as a Silver and not much easier as a Gold. (I can count the number of times I've ever been upgraded on DL on one hand.) For our (especially her) purposes, I'd probably go with AA, but it's not a slam-dunk. I would only fly UA out of BNA under duress--I can't think of any circumstance under which it ever made sense for me.

Now I live in SoCal and don't have to fly for work, so this is not nearly as important to me as it used to be.