Presumably Dulles because international and probably long-haul generally flights. (Though I imagine more fly in and out of Reagan/National on a regular basis.)
Periodically, Congress pushes to have DCA take more traffic total and from a larger portion of the US. Currently, DCA is limited to destinations 1250 miles away, though powerful Congressmen have obtained permanent waivers for this over the years for specific flights so they get an easy flight to their own home region.
My bias is that I think Dulles needs to become a better airport rather than limiting competition with DCA.