I don't have any experience in Korea but if you want to teach in Europe at all, CELTA seems to be the way to go. When I taught in Madrid, I was lucky - the schools and academies were not demanding CELTAs. Since I left, I understand that most of the schools/academies will not consider you at all unless you have a CELTA and a degree. If you are certain you want to make a career out of teaching English, I would invest in the CELTA.
Student or Learner