I work frequently in Downtown DC, and I almost always fly in or out of DCA. Easy choice. Metro is right there, and it's very quick into Downtown.

There is some combo public transport to IAD. You take a metro line way theF out into VA, and then wait for a bus. It takes forever. Your cab ride will be $60. It's a major investment in time. Same with BWI, although maybe SWA has some cheap fares that makes it worthwhile. Same with National Airlines out of IAD.

If in DC proper, DCA is the way to go. Saves 2 hours in unpleasant travel each week, plus the ticket price can be saved in transport costs.