been to both, but spent more time in japan.
japan is, along with iceland, my fave country.
i went in march time, which wasn't too warm - in tokyo at least. tokyo is just a bizare place, utterly fascinating and amazing. go to omotoe-sando and harajuko on saturdays to see cos-play people - they're ace. also go to condomania in shinjuku (i think), shop full of condoms, its a laugh.
i also spent 5 weeks or so on hokkaido (and then spent 2 more weeks elsewhere, travelling around). hokkaido is definitely manageable in the summer, not too hot, and some great hiking (why i was there). sapporo is a realy nice city, not half as crazy as tokyo, significantly more relaxed, and they have a good street beer festival
oh, definitely recommend going to a baseball game. i went to the sapporo dome to watch a game, the stadium is the amazing one where the football pitch hovers and goes outside when there is baseball on... great atmosphere! also the fans are barmy! same at the football matches.
as for travelling i hitched quite a bit, which was very safe and reliable, think i waited at most 20 mins for long distance trips. i also got a ju-hachi train ticket - which gives you unlimited day travel on the local trains. these go at 60kph max, and stop EVERYWHERE! they're awesome. you see all the local people doing their local things - going to work, going to school, going to the shops, going back home, going out, plus you see all the countryside... on one epic train day (i was trying to get across hokkaido) i took 12 different trains over the space of 13 hours. it was sooo good...
loads and loads of things to say about japan - its incredible!
china - ok, i like china, and would like to go back and do more.
night trains are cool, god way to meet people and travel without spending too much money which also avoiding accom costs... i loved the food.
while in china i have eaten:
chicken testicles
goose feet
tortoise
snails
ducks heads
eels
penis
um, the first time i went i did the tourist things - shanghai, beijing, wuhan, xi-an, 3 gorges dam (out the way, but impressive). shanghai and HK, imo, aren't that amazing, just like any other world city. beijing is a little different, more 'chinese' than shanghai somehow. best bits imo is the countryside....
and yes, people are rude and will try and overcharge you, esp in the touristy areas (ime at least)
its ace.