Chinese vs. Japanese

Okay so. I've planned to learn an Asian language, but I'm stuck between 2.

These are Chinese(mandarin) and Japanese. I know its sorta biased, but can you guys give me an indication of which I should learn?

I want to go to either country and teach english as a second language, but which would be more "opportunistic?"

China- blisteringly fast GDP growth and projected GDP leader in 2050

Japan- stagnant economy since early 1990s; aging, shrinking population

One of these languages is going to be better for your career OP.