BWI is much better if you intend to go to the White House / CApitol / Tourist areas

YOu avoid the awful mess called IAD plus you have a 5 min immigration wait at max, plus no shuttles from the plane.

IAD is best if you Fly F (or hope for an upgrade) BWI has no F
IAD has prreflight dining too
IAD had (as mentioned earlier) 2-4 flights (depending on month) so easier for varied schedules