I suppose it depends which one you are more likely to use. Is Japanese easier for Chinese speakers?

文竹姐姐，你能不能教粤语呀！！😍😍

I also want to learn😂 Japanese, I think, now, currently, then Korean.☺️😊

Hello. I applaud you wanting to learn language out of a sense of fun or curiosity. That's awesome and don't let anyone tell you it's a waste of time. I've studied a little of both, and here are a few thoughts: It tried studying both at the same time. It was a bad idea. The grammar was too similar and I got confused. Japan had better English support than Korea so I found travelling is easier in Japan when you don't know the language. I enjoy K-drama and anime, and I would like to improve both languages so I can better enjoy the entertainment of each country. Think about your interests and future plans. A language will never be useless and it will surprise you the connections with people that you can make. But above all, do what your heart wants to do, and you won't have to work at it.

If you have to ask, then probably neither. You should have a purpose for learning a certain language in order to succeed in it.