Learn Chinese if you wanna learn Japanese someday. You won't regret it. The katakana(or was it hiragana?) alphabet for Japanese comes from Chinese. In fact, I don't know Japanese but I could survive for two weeks at Japan - you can pretty much figure out wtf those words are on the signboards if you know Chinese. Most of the similar-looking words have the same meaning. Also, ribuck has a good point there. Many countries are starting to set English as an must-take language in addition to their native tongue.
So ... first, Hiragana and Katakana are the two parts of the written Japanese language that do not
come from Chinese (at least not directly). Both are in fact phonetic alphabets. The part of the written
Japanese language that come from Chinese are the Kanjis.
Second, I disagree that learning Chinese will help you with Japanese. Yes, knowing Chinese Hanzi will help you
figure out what's written around you in Japan, but the languages themselves are *fundamentally* different,
in spite of the fact that they use (more or less) the same ideograms.
Quote
Friedcat has a point there - Chinese is damn hard to pick up if you don't have a base. However, most of the kids over my country learn 3 languages here, and some Indians even go to 4 as they learn Indian as their native language. I'm sure you wouldn't have any problems picking another language up. 
Yeah, well.
It depends on your age.
Picking up a new language after 30 is damn hard.
OTOH, if you're in your twenties, you ought to be good.
Best way to learn Chinese: speak it. Even better: find yourself a Mandarin speaking girlfriend.