The way I view my jersey buying is: When this player is gone, will people remember him?
My first jersey was a black Konerko. Then I got a road Jenks. I've been wanting a white Danks for years, but was always afraid he'd get traded. Also, he hadn't been around long enough for me to feel comfortable that people would remember him. He's been spectacular this year, so I'll probably pull the trigger sometime soon.
Unfortunatly for the Nats, it seems like there is no franchise player like Konerko on the team. Zimmermann may be the closest and you already have that jersey. Strasburg may be a good choice, though I would make sure he doesn't flame out. Also with that, I know I like to have jerseys that everyone else doesn't have. That's why I never even considered a Beckham or Quentin. (Konerko being a small expection because it was my first and because I really like Konerko). Maybe you're the same way.
I think the short of it is, there is no really same Nats jersey right now. Either take a chance or hold off.