Which country is more stressful? Japan or South Korea?
Both countries are pretty high stress, on different aspects. I'll explain a bit further. And no, it doesn't have to do with North Korea either.
The stress comes from the excessive competition of our society. Everything is like a high strung competition here, from education, college admissions, employment, etc. There's a high emphasis on "face" as well, so there's a subconscious competitive spirit in work culture as well - who works longer hours, for example.
Similarily, everything is fast-paced. People rarely get a break from doing something, whether it be working or partying. People are pressured a lot to follow whatever community is doing, whether it be following a trend or joining a party.
While Japan has a lot of competition as well, I'd say it's less cutthroat in that aspect compared to Korea. Instead, a majority of their stress comes from following societal norms and fitting in. Although Japanese society is famous for its politeness and adherence to society, this comes at a cost as some stress to the people.
It's similar to the "face" aspect in Korea, but at even higher standard - while the face aspect in Korea mainly comes in work culture, in the case of Japan, it's in every action. For example, being calm and put together in public is emphasised in Japan - you never see someone fight or shout out loud over there.
In the case of North Korea
To be honest, the stress are more or less equal in both countries, at least on the civilian level. Government level, I'm not that sure.
Both South Korea and Japan use North Korea as a way to gather public attention - both governments use it as a way to turn people's attention from the latest unpopular policy or gather support for controversial ones; for example, while Japan often uses it as a way to gather support for their nuclear policies, while Korea uses it to justify the US soldiers in Korea.
North Korea has been known to act aggressive to both of us, but we're also quite used to it and know that it's mostly all bark and no bite. It might differ on government level (maybe the government stresses), but the Korean population general doesn't really care much.