Was going to ask this very question until I spotted this thread.

I've got options of DCA, IAD or BWI for an AA domestic flight that connects in Miami. Will be staying close to L'Enfant Plaza so DCA should be only a few stops away for me compared to IAD with the metro/bus combo or costlier taxi option.
Flight will probably be mid to late morning so no rush hour to deal with and the flight costs are the same airport wise.