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Korean is easier by a large margin, and you will find a lot more amicable people there. There's also a relatively sizable Christian minority there, so that's a plus. Negatives include: Proximity to literal hostile communists, extremely common plastic women, and arguably not as cool as Japanese (I think they're both cool but many people don't know more about Korea other than Kim and Samsung)
Japanese is way harder and you'll probably be disliked by people there at least at first, but Japanese is much more useful on a larger scale and you get to watch Animu in their native environment.
Personally, I would pick Korean, purely because it would allow you to better learn Japanese down the line, and you aren't as likely to be seen as just an annoying otaku for as long as you're there, unlike in Japan.
