AS could be a good choice if you can't pick your airline seeing as you can earn EQMs on both DL and AA. Plus, MVP status means something (although not much) on DL and Boardroom membership includes many SCs and ACs.

However, because of UA at IAD, US at DCA and CO at IAH *A probably has the IAD/DCA-IAH/CLL route best covered. I think I'd make an AS account just in case but I wouldn't be surprised if you ended up doing most of your flying on UA/CO.