Reagan (DCA) is much closer than Dulles. There is a Metro (DC's version of a subway) stop that runs from DCA to Rosslyn, which is across the river from Gtown. From there, you can take a free shuttle bus to the school. Conversely, you can take a cab from DCA to a hotel in Gtown or Rosslyn for $10-$20. Total travel time from the airport will be about 10 mins.
Dulles may be cheaper (and that is not always the case, if you spend a little time looking for good fares to DCA) but it is much less convenient to get to GTown. From there, you can take a group shuttle (no idea on the cost) or a cab (VERY expensive.) Or, you can take a city bus that runs from Dulles to the same place in Rosslyn where the free shuttle runs to Gtown. However you do it, you're looking at about 45 minutes on the road *if* (and that can be a BIG "if") there is no traffic.
Good luck on your interview!