I spent 3 years learning Mandarin in college, and then spent a full 13 months at Beida in 96. I've had significantly less training in Japanese: one year in CC, but decades now of regular subbed anime, a Japanese wife, and two Japanese-speaking-first children. I think their relative complexity is broadly equivalent, but in different ways:

Phonologically, Japanese is easier. No tones, and no retroflex consonants to worry about as in Mandarin (unless you don't mind sounding southern/Taiwanese). There is a pitch accent in Japanese, but apparently it varies wildly from one end of the country to the other, and it's not generally critical to comprehension, unlike Mandarin tones.

Lexically, Japanese is easier, at least IME. Don't know a word? Given all the borrowing in Japanese, I've had very good luck just to mangle the English word into Japanese phonotactics, and 90% of the time, the person I've spoken to immediately knows what I mean.

Syntactically, there's a lot more similarity in Mandarin's basic SVO word order to English.

But Kanji. OMG. You need to learn roughly the same set of Kanji as hanzi to be high-school literate, but the Japanese set will generally have a minimum of two pronunciations per character, which vary wildly in context. Mandarin generally has one pronunciation per character, which becomes far simpler to learn.

According to the Foreign Service Institute of the U.S. Department of State, Chinese and Japanese are in the same category of difficulty (2200 hours).

But, there's an important distinction. Japanese is singled out as "somewhat more difficult for native English speakers to learn than other languages in the same category."

From the contact I've had with (Mandarin) Chinese, the basic grammar has the same word order as English. You have to teach yourself to interpret tones as lexical information, instead of just emphasis/emotion/flow. You need to learn more characters in Chinese than in Japanese for equivalent proficiency. Deeper into the language, there are more "two birds, one stone" kind of metaphors that are widely used (basically, cultural points that you need to learn, even if you understand the literal meanings of the constituent words).

Those are a mixture of my impressions, and the opinions of friends who speak the language (both learned and native). Overall, I think the most difficult part of mastering Chinese and Japanese is the same: Lack of shared cultural context, if you come from an English-speaking or European country.

Tones are tricky in Chinese, on the other hand, there is no conjugation which I really appreciate.