Japanese, because Chinese is a lot more challenging:

• In writing, exclusively complex characters (hanzi) are used. You'll need to learn a few thousand of them to be able to even read an ordinary newspaper.
• In speaking, you have to differ between four different tones, and changing the tone totally changes the meaning. Also, there is a plethora of rules governing the pronunciation of how tones interact with each other.

-> This makes picking up Chinese by yourself near to impossible. You should only pick it if you have a lot of time and a good, professional teacher.

Learning Japanese is still quite time consuming, but not quite as challenging:

• The writing is a mix of complex characters (kanji) and simple ones (kana). You can get by nicely already by knowing the kana and the few hundred most used kanji.
• The speaking is quite simple, as there aren't any tones or other difficult to produce sounds.

-> Thus, picking up Japanese by yourself isn't impossible - but still hard. I'd recommend taking a course at your uni if you're serious.