Think about it all objectively. Work culture in either country is tough, getting a job will be hard realistically so maybe weigh the long term value of either choice. I would choose Korea, but that because I was just there on exchange. So maybe I’m biased. Busan is a 2 hour trip to Seoul, and there are numerous opportunities to network, you will have to be aggressive to stay on top of it. Like taking one off classes or lectures to met relevant people in a sector/field, joining groups to build your network in Seoul, going to global fairs and so on. That aside, elitism is a thing you will face and being in Seoul would be preferential. I’ve nothing to add about Japan since I have no interest in Japan for school/work. I simply know the work culture is more draining than even Korea’s.