C-3-8 is exclusively for the Korean diaspora, while C-3-9 is for everyone or everyone else. However, both of them are valid for 5 years, are multiple-entry, and allow a maximum stay of 90 days. Is there any difference in practice?
The difference is not about the benefits, but about eligibility. The C-3-8 visa is race-based: it is given to Overseas Koreans, ie Korean by blood, but with a foreign passport. The criteria to obtain C-3-8 are much less stringent than C-3-9 or C-3-2.