I vote for the train, too, and I live near Baltimore. From Penn Station you can take the Light Rail downtown to the convention center or Camden Yards, or you can catch a bus that will probably take you directly to your hotel (which one?). The light rail leaves Penn Station every half hour at that time of day. Or you can grab a cab at Penn...it's not too long a drive.

The light rail also goes all the way to BWI, so you can catch light rail from there and end up stopping at Camden Yards and walking to your hotel, if you're only carrying an overnight bag. Note that a section of the track between BWI and the city is currently being worked on, and you have to get off the light rail, take a shuttle, and get back on the light rail again. Not convenient.

I must say, though, taxis from BWI are good. They're strictly airport taxis, and while expensive, they know their way into the harbor. I don't think taxis at Penn Station are as regulated.

Visit MTA Maryland for good info about getting around Baltimore on public transportation.

--Chris