BWI is indeed cheaper to fly to. As for distance from DC both Dulles and BWI are really equidistant. But BWI does have a direct rail connection, both Amtrak and Marc, plus Baltimore Metro light rail all have connections to BWI. There is now a lot of talk of extending the Orange line from Vienna out to Dulles, but that will take sometime to complete if and when it is approved. Plus BWI has a much larger population cachement in its favor, not only DC and Baltimore but much of Maryland and parts of Delaware and Pennslyvania. However the population growth in Northern Virginia is much faster than in Maryland.
But the short answer is that really all things are equal, but BWI is cheaper.
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia