MARC Penn Line from BWI or Odenton because you'd be so much closer (or there) rather than fighting your way up 95 from Greenbelt in rush hour, but to get to Nationals Park from Union Station it's just a little too painful and the train times aren't so hot so if you didn't get out of your stuff right at 10, you'd be waiting another hour to try again and be 20 minutes late to the game. You could go from Odenton, which would be a safe bet time-wise (and let's you still avoid 95 in the evening), but a risk of no parking
Here's what that would look like for you: https://goo.gl/maps/QVtSofiFCxF2

The payoff is that it's probably quicker and less stressful than taking metro back to greenbelt (about an hour) and then driving 95 in rush hour. When you'd be heading back there are more MARC and Metro trains so it's less risky that you miss something and it takes an extra hour.

All that said, unless something about that stands out to you, I'd probably just do Greenbelt like they suggested.
[Post edited by jmanatVT at 04/01/2019 4:17PM]