BWI and Dulles are roughly equal in the number of passengers served they differ by under 2% and I am not sure which is busier. BWI is great to fly out of. Traffic in the BAlt/Wash area is pretty bad in general so it can be a chore getting to it from NoVa. However from my place in Baltimore I can get to Philly faster than I can get to Dulles in even moderate traffic - but I am 10 minutes from BWI so that pretty much sums up my airport choice! Easy departures is BWI's strong point. The terminal is pretty well laid out and very accesible Light Rail and train to Baltimore train to Washington. Baggage claim at BWI is a nightmare - perhaps the slowest in the US. However Dulles has those crappy trams between terminals this alone makes BWI superior - even if it takes you 30 minutes extra to get to BWI v Dulles you can make up the time right there. Of the three airports it is hard to say which is best as I said before traffic can be horrendous skewing the equation. One thing is for sure if Baltimore is your destination Dulles is out.