DL is nice because they're giving away a lot of miles through bonuses these days, but beware that you might have a harder time redeeming DL miles than other program miles.

AA doesn't have much presence in BWI, but it has more presence in IAD and especially in DCA.

As for AA challenge, just keep your eyes peeled. Sometimes AA will run pretty cheap W fares to, for example, Europe. Or the recent H fare for EWR-LAX.