Both Urdu and Bengali are major languages in India as well.

 I was referring to different languages. I think most languages have different dialects.

If you're only interested in what would be the most useful for future career prospects etc, then Mandarin Chinese is probably the thing for you. It's spoken by most of the country, especially all the "important" people, and China is definitely rising as an international power. There are some human rights issues, but the situation is slowly getting better, and they're probably a lot more careful with how they treat important foreigners than their own people.

I plan to learn Mandarin and maybe Arabian later on, but chose to learn Japanese first due to several other interests in the country and its culture. Japan is clearly exporting the most popular culture at the moment, but even if you don't care for that (your Naruto avatar and sig pics suggest otherwise), it's still useful in business. Not the most useful ever, but definitely better than for example Finnish or Swedish. =P

While all three have lots of smaller dialects, sub-languages etc, India is by far the most confusing for an outsider. IIRC the biggest local languages (Bengali and Hindi) are spoken by only a third or so of the population, and most of the educated people are pretty good with English. They'd probably appreciate it if you knew their language, but know damn well they aren't important enough to expect it.

i'm learning chinese, not because of its importance, but because i like it

I'm starting my A2s next year. I'm taking Computing, Mathematics, Government and Politics at A2, and Economics at AS.

To answer your question - I'd seriously suggest going to the countries first. Or, perhaps, learn another language that is spoken in more countries. French, German and Arabic are all spoken in more countries than Chinese. Seeing as you're from the UK, perhaps you could consider learning some Eastern-European languages.

Chinese - most spoked language on earth! (Mandarin)

Manderin Chinese is what you need. Japan's economy since the 1990's has been in stagnation. Indian there are way too many dialects and thanks to the Brits English is very widely used in that country. Manderin is the way to go mate.

I speak both, (CHinese and Japanese), my Chinese is by far better of the two.

In my opinion, learn Chinese, it's slightly easier and way more people speak it... Unless you want to play JRPG games, then you learn Japanese.

Learn chinese, it's more important, and for what i know when you learn one you are halfway with the other because of the kanji (is that right?)

But i couldn't agree more with SamuelRSmith. I suggest you learn some east-european languages, i think they are more important.

 well, not halfway, but there are some similarities. Much like English and French.