Japan by a mile. Don't get me wrong, Chinese culture is fascinating, and there are many historical sites in China which are must-sees. The Chinese government is despicable, though, and the sheer amount of pollution in the larger cities is unbearable. I've met plenty of friendly people from mainland China, but when it comes to mainland tourists... let's just say I keep my distance.
I'd say that Taiwan vs. Japan would be a more balanced match-up, but maybe I'm just biased.
China. Especially for me ROC is the real China. The PRC regime, as much as I back them on an issue by issue basis, are a bunch of illegitimate interlopers. I am actually fairly pro-Japan within the Chinese political context but between the two I have to go with China.
