Mandarin Chinese for sure! It's the language of the Future!
Chinese and Japanese both are mostly same, so if you want to learn chinese so japanese will very easy for you and that's why if you learn japanese language so chinese will easy for you so that's why both are same and interesting languages, but I think chinese is not the future language because english is using for world wide and everyone can easily learn it in schools so its very difficult for us.
Chinese is right now more important in the World than japanese, and in so about10 years every good educated person will speak Chinese!
Japanese is very interesting language but it will never become that important as Chinese!