South Korea is more technologically advanced and developed. They also have a relatively large military
SK has been preparing to defend militarily against the massive NK military for decades. Japan is demilitarized with only 300,000 troops aprox. SK would probably win easily
Nihon ni Katsu
The sushi is better there
North Korea would help japan, Forcing Chkna to as well.
Koreans might have better tech, but Japan has a larger army currently, more available applicants, more weapons, resources, population and size. Plus the Japanese have a clearer shot at hitting Seoul than the Koreans have with Tokyo.
Japan, they have better tech (besides nukes) and a larger standing army, and even if south korea wanted to, they don't have the man power to invade
i will vote for japan.