Hey there. And warm welcome to linguaholic!
Not easy to give you some recommendations here but I will try. After reading your post, this is what came to my mind:
As your native language is English, studying German will definitely not be that difficult. You will already know lots of words because English and German obviously share lots of Cognates. German grammar is somewhat difficult and definitely more difficult than English Grammar. But still, nothing too crazy, there.
Japanese is definitely a very interesting language to learn. And in your case, as your goal is to move to Chinese later on, I can tell you the following:
As far as speaking goes, studying Japanese will NOT be of any help to learn/understand Chinese later on. Well, there are Chinese pronunciations of Japanese words but to be honest, those are not even that close and unless you are very advanced in Japanese, you probably won't even hear the similarity. Moreover, Japanese is not a tonal language and Japanese and Chinese are after all NOT related etymologically.
However, as far as Kanji go, those will definitely help you to master Chinese later on...as guess what, Kanji are almost all derived from Chinese. However, there is one problem: The Kanjis used in Japanese are still in the traditional form (= traditional Chinese characters). So that means that you would learn the complex versions of the Chinese characters, which are now only used in Hong Kong and in Taiwan. On the mainland, people use simplified Chinese characters, these days. That is not a big problem but definitely something you should be aware of.
In terms of Grammar, studying Japanese won't be of any help either.
To wrap things up, I would just like to say: I think it is always best to study the language that you have some passion for. If you don't have any passion or at least strong motivation to learn a language, chances are that you will never get anywhere. This especially holds true for a language as"exotic" as Japanese or Chinese. Learning these languages will take you YEARS and it is just simply not something you can do half-heartedly.
If you just feel "forced" that you have to do one of those two at the moment (German or Japanese), studying German will definitely be the easier choice for you. If you go with Japanese, you will be able to pick up some Kanjis that can be very useful for your later studies in Chinese, though.
Those are my thoughts about your question. I could go on for another half an hour, I think. But it's probably better to stop here for now. If you do have some specific questions about either German or Chinese (German is my native language and I studied Chinese for many years), feel free to ask.