Trust me, I live across the river from Georgetown and often work at a client site near Union Station. The Circulator bus may be "easy" but it is not a crosstown commuting option, especially when you are following it up with an hour-long train ride to Baltimore. It will easily take 45 minutes to get across town during rush hour (I think the D5/D6 Metrobus would be a similar story). Somewhat better might be to take the bus to Dupont Circle and try to board the Red Line there.

Actually, I forgot that there is also the MARC Camden Line. The schedule is much more limited (only 2 southbound trains in the late afternoon, 5:15pm and 6:10pm), but it might be feasible for someone working a traditional schedule near the Inner Harbor.