Is there any way you could perhaps take a course in which ever language you decide to go with? With a decent teacher, you will always learn a language more quickly and comprehensively than if it's self-taught. Assuming that the teacher is from the country that the language is spoken in, naturally. That sounds like a given, but my sister spent the last few months being taught Spanish by a Romanian at school for some reason.

Anyway, I tried to teach myself Japanese once, but I found it extremely difficult. I'm now doing Japanese at university (not as my main degree. For credit.) and it's going much better. I suppose if you wanted to get a head start with Japanese before taking a course or whatever, you might try learning Hiragana and Katakana and a few Kanji, but I wouldn't recommend going any further than that.


-EDIT-

Okay, just realised that your main query was as to whether you should go for Japanese or German. That's not the kind of decision anyone else should make for you, really. The best you can do is research the culture and language of both and then make an informed decision. Personally, I'd go for Japanese first, but then, I'm a Japanophile. In which would you rather life? Would you rather date a Japanese person or a German person? It's all about personal taste. And I understand that those are rather specific and not necessarily applicable questions, but they are the kind of questions that may help you to reach a decision.

That said, I'm hardly an expert on linguistics. That's just my opinion. Good luck!