Go to China, as it's changing very rapidly and you can see a culture in transition, plus amazing hustle and concentration of activity; Japan is not changing at all and will be just as beautiful and the people just as nice a decade from now. Japan is for the long term and any time, while you should visit China often.

English is better in much of China, in urban areas, but the service almost uniformly bad, so Japan will pamper you if you are comfortable with it and can get around on your own without stress (though harder to travel in China, neither is easy for the newcomer).