Well, I'd say you could be politically active where ever you are going, but obviously if you do choose NY, it can't be on the same scale you'd like it to be. But you can always go back to DC. You're young.
Obviously, you should probably wait and see what type of feelings you develop for this girl (you already have some anyway).
I hate to say that you should just go with the flow, but it sounds like maybe instead of jumping to a decision right away, you can go with your gut, considering neither of your options are really going to hop on the next flight to Europe and disappear on you.