I would go with CO, mainly for its generous domestic upgrade program. As far as I know, there is not a huge population of elites in the DCA area. Once you get to Gold status, you should not have a problem in getting upgrades. The one caveat is that you should also look at what you intend to spend all the miles you accumulate on. If it is for upgrading international flights, CO would be a poor choice (see the ongoing thread on the CO board). If you want to go a lot to Asia UA has the best network there on its own metal. AA can get you only to TPE/NRT/KIX on American metal.
I would simply suggest that what you intend to spend the miles on should be one of the most important parts in your decision.
Good luck.