Been to both about 3-5 times... and find them both equally fascinating. If i had to chose one myself to go to, I'd probably pick Japan, as it's a bit less bureaucratic to get to. I'd also note that the scenery is rather better in Japan, as a) the inner mountain area is less developed and b) the communists didn't really care about destroying nature in the name of progress - so despite having been to several provinces, I've yet to see anything I could actually look at and go "that's pretty". As a birdwatcher, the long scale effects of various agricultural schemes is still working it's way through the system too, and in much of the east, heavily populated areas of China, the ecosystems are pretty badly messed up, with agricultural production, hunting, over fishing and pollution. There are exceptions obviously, and some really great birding spots, particularly in the south....but way off the usual tourist tracks.