Mandarin or Japanese!?

Since you already know basic Japanese, suggest you take Japanese in college, just finish something you already started, you can always learn Mandarin later. As for usefulness, the reality is China is becoming a major economic power, many foreigners would like to do business or study there. At the same time most Chinese people are eager to go overseas and connect with the outside world, therefore almost every educated Chinese person are learning English. As of today within big cities of both countries, it's easier to find some local people speaking English inside China than Japan.