China had not annoyed Koreans with the comments of their forays into the Korean War or other previous encroachments while Japanese political establishment has always made a point to show their view that the Japanese occupation of Korea was OK. So China has a slight advantage now. But that could disappear if Chinese would become overbearing. If a Korean is left with 10 Chinese, Chinese usually get bold and say things like, "Korea should have been part of China a long time ago. Why are you still holding out ?" Their overexpansive nationalism is something to worry about.
I do not think that China would keep growing as she is now. Both Chinese and Japanese economies will collapse at some point in the near future. Japan will turn more aggressive. China could turn aggressive but she could be also split up in the ensuing chaos. If these happens, Korean economy would suffer, too.
Who will be the more threatening to Korea will be difficult to tell. If neither is particularly stronger, Korea may stay neutral.
Regs,
VRN