It really depends on the level of interest in the cultures of both. If you enjoy anime (or the like), go for Japanese. If you want more $$$ potential in the future, maybe Mandarin is a better investment. Japanese culture is more visual and people seem to be more friendly at first, so it's easier for foreigners, especially the white gaijin, to fall in love with Japan easily. Chinese culture is something that takes time to be appreciated, and Chinese may sound a bit rude or look a bit cunning at first, but once you've gotten past the superficial, things will look differently I am sure.