I have studied both Japanese and Chinese for quite some time. I never reached any fluency but I can communicate on some level in both Chinese and Japanese. I can also read in both, I hate writing by hand but I can send emails in Chinese and Japanese.
I've come to the conclusion that Japanese is easier when you get to an intermediate level.
Japanese difficulties:
1. Grammar, sentence structure.
2. Inconsistent, often unpredictable kanji readings sometimes ridiculous readings, e.g. you can tell readers to pronounce
ヒ?ェナ?pツ獣 (ikkakujū) as ニ停??ニ男ニ坦ツーニ停? (yunikōn). Therefore many books are supplied with furigana (ruby).
3. Speech levels - formal, informal, casual, honorific cause problems even for native speakers. Manga Japanese (extremely casual and often distorted, peppered with dialectal forms or words) is hard to read.
4. Heavy use of onomatopoeia to describe not just sounds but many other things. This concept is different from many other languages
Chinese difficulties:
1. The number of hanzi is significantly larger. Whatever you read/write, has to be in Chinese characters, including your own name, any foreign place name. Loanwords are few and they don't make your learning any easier. Their pronunciation is much more consistent and predictable than Japanese counterparts but a large percentage of Japanese text is hiragana/katakana.
2. Simple grammar is not always a blessing. Long sentences are sometimes harder to understand and break up.
3. The number of ツ青ャ语/ツ青ャナ津ェ (chéngyǔ) is high. The Chinese idioms are not easier to understand, you need to learn them.
4. Tones are hard. They are not a show stopper, though. You need to spend weeks or months, not years to learn them but I know a few people who are terrible with tones, even if they work hard.
5. Listening comprehension is much harder in Chinese because many words are shorter and their meaning depends on the context. It's hard to pick up "chengyu"
Overall, when reading in Chinese, I have to use a dictionary quite often and you usually need a dictionary, which allows to draw characters. Luckily, there are good dictionaries now.
Japanese seems easier to learn from picking up conversations.
All depends on your interest and ability, of course but I find Chinese is objectively harder. I have difficulties with casual Japanese but I can slowly read modern stories in standard Japanese. I have spent more time learning Chinese but I still can't read Chinese books on the same level or it takes about twice as long.
I have tried to be objective, didn't mean to criticise Chinese. I like both Chinese and Japanese and will continue learning and using them.