In German, you would have to learn a lot of irregular noun cases. That's certainly very difficult. In Japanese, what is equivalent to cases is expressed by adding particles after the noun. read more
As German is my native language and since I have just begun to learn Japanese, my answer might be a little biased. I think German is more difficult to learn, except you have a strong background in similar languages like Dutch or Swedish for example. read more
Although German also have a set of formal and informal speech separately like Japanese, the language itself is a whole lot easier, as well as will likely serve to be more useful to study than Japanese, as it is more commonly spoken. In conclusion, Japanese is more difficult to study than German. read more