I spent a year playing silly games and acting like a clown. There is no doubt that Japan is superior in this respect. People are almost always extremely kind, even if they hate you. On the other hand, they all made more money than I did and I couldn't afford to go out much. Try that in Japan and see how fast Y20,000 flies out of your pocket.
Japanese cities are usually very clean, unlike Korean cities, with the notable exception of Seoul.
The water is drinkable and eveything works perfectly. On the other hand, Japan can be extremely dull and boring as every town looks exactly the same as the next and the people are usually reserved, shy and quiet. At present, Japan is saturated and Korea is screaming for teachers. I honestly think that if you look carefully and avoid head hunters you can land a good deal in Korea. Most of the really shady hagwans are gone now.