Not yet mentioned on this thread: MARC doesn't run on weekends. Otherwise it is much quicker than the Metro setup. There's still a short bus ride, shuttle bus from BWI Rail Station to the terminal, but that's just maybe 5 minutes.

On weekends the Metro gets more appealing because while it's still slower it's much cheaper than the Amtrak train which is the only Union Station-BWI Rail option running on Sat and Sun. (The bus is cheaper than MARC too, but only by a couple bucks, so not so worth it.)

BTW, you want the Penn Line from Union Station, not either of the other two MARC lines that leave from there side by side. (And, occasionally, stacked two deep on the same track, from what I remember.)