I'd consider AA as an alternative since they have so many nonstop options out of DCA, including many of the routes you'll be flying. DL will have you connecting in ATL a lot - which is not the end of the world, but will suck so many hours out of your life over time that it may not be worth the better service experience and arguably higher reliability on DL.
UA is only worth considering if IAD is convenient for you - out of DCA they tend to offer even fewer options than DL, typically routing you through ORD (yeesh) or IAH. They have a very nice mix of international partners out of IAD (LH, NH, TK, etc.), but their own concourse at IAD is a total dump.