There is only one good advice :-) if u just wanna learn those languages for fun,read the other advices ...
if you seriously wanna learn a language: VISIT THE COUNTRY FIRST !!! Most germans(students) first study chinese for 2 years and than come to china for one term ... afterwards near 40 % are quitting their chinese studies ... so a waste of time ... i travelled china and japan and decided to learn chinese :-) ... and: you can learn every language if u r in the country

if u are really smart, u  can learn chinese, even though i'm a chinese , my major in university is education about teaching foreigners chinese.   chinese is my mother tongue, i think it's really hard, especially  ancient  chinese part. 
      and i can speak japanese too, i think japanese is more easy for chinese , it's easy to learn at first, but a bit hard at last.  anyway, it depends on ur interests, if u really like chinese or japanese, u needn't hesitate.
     gud luck!

Chinese
You will see how interesting it is.

my experience with japanese is very limited but i think it depends on you. The tones in the chinese language  make it seem very difficult when you start but after you study for awhile learning chinese becomes easier. (the grammar is not so difficult) I think japanese is easy to start but gradually gets harder and harder. Both languages are beautiful and what ever culture you like best is probably the one you should go for.

Japenese is a easier choice,especially a nicer one for OTAKUs.