I've done the IAD-JFK commute for several years. Overall it's pretty crappy: Jetlue is down to only 2 flights a day with E190s. Everything else is United Express and Delta Connection CRJs that go out full to the gills. You'll have way better luck out of DCA, what with the USAir and Delta shuttles to LGA, along with a good amount of dca-JFK stuff (i believe Delta was even running a 319 at one point.)
Good luck, I may see you on that commute with the next bid run.