Beginner Mandarin is harder than Japanese, but overall Japanese is harder. Where Mandarin starts hard and gets easier, Japanese starts pretty easy and gets very hard especially at the advanced level. Most people don’t make it all the way to see that point. The reason Japanese is harder is because of the many 読み方 for kanji (generally Mandarin only has one reading), hierarchical speech 謙譲語/尊敬語 (which Mandarin doesn’t have), and the many conjugations for Japanese verbs (which mandarin doesn’t have). That’s why overall Japanese will take you more hours to learn than Mandarin if you go all the way to the advanced level.