My wife is South Korean so I'm biased. But I've been to Japan and South Korea and if you're looking for one of the most modern nations in the world, I strongly recommend you go to Korea. The only comparable country I've been to is the Netherlands or maybe Denmark. In contrast, the US and Japan seem to be somewhat falling apart with a peak in infrastructure and buildings a few decades ago. It's not that bad of course…
I felt the same way about Japan when I first visited it in the late 1990s. Japan has stagnated badly since then, and due to the demographic crunch, it's essentially certain that Seoul in 2040 will look like Tokyo in 2020. (Unless North Korea transforms it into a smoking crater in the ground, that is, since the entire city is within artillery range of the border.) Also, in my experience, Koreans speak even less Englis…
One more massive advantage Korean has over Japanese is that Korean has a fairly simple alphabet so you can read and pronounce words after only a little practice.
In contrast, it takes quite a while to be able to read Japanese with their three writing systems and Kanji in particular.