You didn't say what day of the week you'd be arriving. If it's on a weekend, MARC runs much less often (though at least it runs, which wasn't the case until last year or so) and you can be stuck waiting on a more expensive Amtrak train. Speaking from personal experience, the BWI rail station is not where you want to spend much time.

But the B30 bus is no prize either, since it takes a long time to get to the Greenbelt station--you will probably feel like you've gotten hopelessly lost as the bus rolls through the wilds of the Beltsville Agricultural Research Center--and the ride downtown from there isn't fast either. Especially on weekends, when Metro rebuilding can impose 24-minute headways on any given line.