Mandarin of course. i say so not because i am from china. in fact,japanese and korean are from Mandarin,and koreans and japanese used to speak Mandarin.in the 20th century, good koreans and japanese scholars are asked to study Mandarin well. When you have grasped Mandarin,it will be easier for you to learn Japanese. Just like that when you have grasped English,it will be easier for you to learn French.So enjoy your learning Mandarin,sukizo!