Realistically, I won't ever get to play the other. I can only play this one. Yeah I'm not 100% sure about that, but let's just say for the purposes of this topic, it's true.
If you'll only ever get to play one, go with BW2, far more content, you can look up anything you miss as far as storyline goes, many more pokemon and much more bonus content, Joint Avinue, the movies etc. It's just all around the better game.