Can't decide between learning Korean or Japanese after learning Chinese (Mandarin).?

You've already got a good character base from Chinese to help in Japanese. That said, they've got multiple character bases. Korean is super easy to learn to read and write. Can pick it up in an hour. But the grammar in Korean is mind numbingly hard. Anyway, I'd pick the one that you think you're going to end up using more. If it's for money / future career though, Japanese is better because tons of Chinese and Koreans speak each other's language but very few Chinese and Japanese speak each other's language, so it's a better payscale for that skill.

Take your time and pick the one YOU prefer, no matter which other people could prefer.

Korean and Japanese share some grammatical features but I think you should go for Japanese because you know some kanji already

Now learn cantonese.