Mandarin Chinese or Japanese?

Which do you think is the most difficult to learn for English speakers in terms of writing, reading, speaking, and understanding?

Some people who studied Japanese say the numerous alphabets of Japanese is pretty challenging. For Chinese-learners, many say that the tones are the most difficult to master.

The Foreign Service Institute (FSI) of the US Department of State has compiled approximate learning expectations for a number of languages. Of the 70 languages analyzed, the 5 most difficult languages to reach proficiency in speaking and proficiency in reading (for native English speakers who already know other languages), requiring 88 weeks, are: "Arabic, Cantonese, Mandarin, Japanese, Korean", with Japanese being the most difficult.

So the answer would be Japanese.