Is it easier to learn Chinese after learning Japanese or vice versa?
I think it would be easier to learn Chinese after learning Japanese rather than vice-versa, because too many Chinese characters are used in Japanese. I would like to talk about this from three points.

Pronunciations

Usually, in Japanese, one character has 2 types of pronunciations, &quot;音読 おんどく ondoku or 音読み おんよみ onyomi&quot; and &quot;訓読 くんどく kundoku or 訓読み くんよみ kunyomi&quot;. The 音読 (in Chinese, 音读) is similar to the pronunciation of the character in Chinese, because both modern Chinese and 音読 come from ancient Chinese (漢字を字音で読むこと。). 訓読 is the Japanese way of rendering the characters pronunciation, choosing the similar Japanese vocabulary in meaning to substitute (訓読は、漢字を、その意味にあたる日本語の読み方で読むこと。). This could be an advantage when you learn one of these two languages after you learnt one another.It can help you to remember the pronunciations.  But it could be an obstacle in some cases, since the pronunciations of characters in Chinese also changed in the long history.
Example: Character 人. In Japanese, its 音読 includes ニン nin (呉) as in 人間の歴史 にんげんのれきし　and　ジン jin (漢) as in 登場人物　とうじょうじんぶつ. These two variants, ニン and ジン, come from different times of ancient China; one of its 訓読 is ひと. In modern Mandarin, the pronunciation is &quot;rén ㄖㄣˊ&quot;, however, I know in &quot;吳語&quot;, the pronunciation is &quot;nin&quot;, which proofs the ニン nin (呉). Anyway, this could also help you to remember the 音読. If you know &quot;Nin&quot; and &quot;Jin&quot; for &quot;人&quot;, you may guess &quot;nin&quot; or &quot;jin&quot; for &quot;仁&quot; (actually, it's right!), because both &quot;人&quot; and &quot;仁&quot; have the similar sound in ancient/modern Chinese.

Writing Characters

Since Japanese uses a lot of Chinese Characters, it's an advantage for you to write down the characters if you have learnt one language. However, you still have to take care of some characters. Because both Japan and China conducted several reforms (i.e. simplification for better literacy, though in China some political motivation too, which is a controversial action) on traditional characters, and of course, they didn't negotiate with each other (though in fact, Chinese government did treat Japanese version as reference, e.g. 國 simplified to 国), so some characters are different after the simplification. One thing is for sure, simplified characters done in China is much more than those in Japan or Republic of Korea.
Example: Character &quot;对&quot; in simplified Chinese while &quot;対&quot; in Japanese, the traditional being &quot;對&quot;.

Meanings of Characters

In most cases, the meanings of a character in Japanese and Chinese are the same or almost the same. This is a big advantage. However, Japanese generally keeps meanings of a  character in Classic Chinese, while in modern Chinese meanings of some characters have changed. In this case, the learner should pay extra attention to these characters, compared to a learner who doesn't know one another language, because his experience will lead him to a mistake.
Examples: Japanese verb &quot;走る はしる hashiru&quot;, which means &quot;to run&quot;. In modern Chinese, we use &quot;走&quot; to mean &quot;to walk&quot; and use &quot;跑&quot; to mean &quot;to run&quot;, however, in classic Chinese, &quot;走&quot; really can mean &quot;to run&quot;, as in 戰国策·楚策·江乙對荆宣王:
荊宣王問羣臣曰 吾聞北方之畏昭奚恤也 果誠何如 群臣莫對 江乙對曰 虎求百獸而食之 得狐 狐曰 子無敢食我也 天帝使我長百獸 今子食我 是逆天帝命也 子以我為不信 吾為子先行 子隨我後 觀百獸之見我而敢不走乎 虎以為然 故遂與之行 獸見之皆走 虎不知獸畏己而走也 以為畏狐也 今王之地方五千里 帶甲百萬 而專屬之昭奚恤 故北方之畏奚恤也 其實畏王之甲兵也 猶百獸之畏虎也
Compared with the advantages and disadvantages, I believe it could be easier to learn one language of these two after you have learnt one another.