EDIT: just to be clear by "you" I basically mean historians in general, not "you personally". I don't want anyone to reply and tell me that for them it's Japan because they happen to own more books on Japanese history
Absolutely and unequivocally: China. We have histories of the period, poetry, a mountain of primary sources dealing with civic society, religion, philosophy, you name it... For Japan at the time we have the wei zhi - a 3rd century history...of the Chinese Wei kingdom, that happens to include a description of a diplomatic visit to what was most likely (but not definitely) the two main islands of Japan. Kyushu and honshu. There is just no comparison.