I already speak Japanese fluently and I want to pick up another foreign language to fluency level.

Which language would be more useful in the future? Chinese or Russian?
Thanks for your input everyone! I appreciate your responses.

Louis the Linguist: I "picked up" Japanese and I know the effort involved. I've studied the language for several years including spending a couple years in the country. I'm prepared to study another language for years and live in that country for a couple years as well. Don't patronize me!

i want to become a linguist in russian but im unsure if alot of people speak the language and if it would be a popular language to work with in the U.S. i know russia is a big country but so is china (population-wise) which has many languages so again would this be a good language to work with?