[Eomer]

I suppose it depends where you are in life and to what level you want to learn.. if you are at school, have time and opportunity, I would pick Chinese first.

The reason is because after High School/College/Uni you might wanna go live in one of those countries to help you learn faster, and living in a foreign country is the best way to learn any language. So with that in mind, you should know, that living and working (as an English teacher say) in Japan, is probably gonna have more benefits than doing the same in China, I mean I'm sure there are great opportunities for work in China, but (and this might be a little biased) I just think there are more here.

Also I should mention that it's not a good idea to try and learn both languages simultaneously, especially in the early stages. IMHO you should wait until you can pass (or nearly pass) the highest level of the first languages' proficiency test before starting the next.