Yeah, Chinese will be more useful when you're job moves to China :-*
Seriously though, more people speaking the language would theoretically make it more worth your time.
I never actually answered this question.
I would go with Mandarin (I think someone mentioned that before). It is the most commonly spoken language in the world. There are nearly double the people speaking it when compared with English (the second most common language in the world).
As a side note, I'm not sure where Japanese falls on the list. I would assume top 10, but my guess is Spanish and Arabic are spoken in higher numbers. Ever considered either of those?