This is an anonymous account, and while I feel that I'd like to contribute my own two cents, I'm not looking for an extended discussion. The American State Department for its own purposes have performed their own research into the expected number of hours needed for their average Foreign Service employee to learn a language. Both Chinese and Japanese are categorized into Category 4, for the hardest tier, but Japanese is marked star, to indicate that it tends to require a significantly greater level of time investment to achieve function or fluency. For the State Department's purposes, Chinese requires 2200 classroom hours, with the expectation of equivalent hours spent studying, for a real investment of 4400 classroom hours, for the native English speaker with no background in Asiatic languages. The star mark, I recall somewhere, indicates that the number should be multiplied by 1.5, so that Japanese would require 3300 classroom hours and 6600 total study hours to achieve fluency.
That said, I do think that certain posters on this forum have overstated the relative ease with which they can learn Chinese. Japanese speakers inherently have an advantage over speakers of other languages when it comes to learning Chinese, as the most difficult part of learning Chinese is based on its orthography, and to have partial mastery of Sinograms means that Japanese speakers have a significant leg up.
The difference in difficulty could be described as that between Romance languages and Latin, with Chinese (and the Sinolects) being Romance languages and Japanese being Latin. The vocabulary is in large part cognate, although faux-amis do exist (the Japanese for letter being the Chinese for toilet paper, for instance, although you can easily see the etymology), but Japanese has a significantly more complex grammar, as well as a tendency towards contextual statements (although both Chinese and Japanese can be described as subject predicate languages). The SVO vs SOV thing is also overstated, as Chinese has grammatical forms (the passive construction, for instance, and the 把 ba construction) which use OSV (passive) or SOV (ba construction) forms.
Someone who is fluent in either Japanese or Chinese havs a significant step up on learning the other language due to the Sinograms involved, but the advantage is greater for the Japanese speaker than vice versa.
Fluency targets also vary, for instance, Japanese speakers are similar to the French in that they expect absolute accuracy in pronunciation, grammar, and diction, while many Chinese speakers do not have Mandarin as a first language, and expect significant variation depending on locale (just as say, for English as a non-native speaker, the most difficult aspect of English may be understanding the accents of non-native speakers). The level of Chinese that is necessary for a native speaker to consider you conversant is less than the corresponding level of Japanese. That is something that is important to remember, while you may be more easily perceived as fluent in Chinese compared to your Japanese counterpart, your Chinese could be significantly flawed in pronunciation and diction.
With regards to sinograms, Chinese definitely requires more, but the current HSK standard requires about 2,500 characters for CEFR-C2 equivalent fluency, while the TOP and TOCFL standards in Taiwan require about 3,000 characters. Jo-yo- Kanji requires about 2,000 characters, while with the Jinmeiyo- extension you require about 3,000 characters. For more expanded requirements, Kanji kentei is cited as requiring 6,000 characters, while the PRC Xiandai Hanyu Changyong Zibiao (List of commonly used Chinese characters) uses 2,500 characters on its core list and includes another 1,000 for less frequent usage. The extended Xiandai Hanyu Tongyong Zibiao uses 7,000 characters, which is slightly higher than the Kanji kentei standard. If you'd like to focus on traditional Chinese, on the other hand, Taiwan uses 11,149 characters on their most comprehensive standard, with about 4,800 characters considered to be in common use.
For usage cases, of course, Chinese by itself is probably a more useful language, given the vast number of speakers, the ability to "fake" CEFR A1-or A2-equivalent functionality in Japanese through Kanji usage, and is also easier to learn. However, if you're interested in both languages, learning Japanese first will save you time if you aim to achieve fluency in both languages, due to the more beneficial overlap from Japanese to Chinese. Chinese mastery is not without its own benefits, however, as you will likely be familiar with generally obscure Kanji that the vast majority of Japanese speakers will not recognize.