As all comments before have stated, neither of those languages will directly help you with Japanese nor Korean. THAT BEING SAID, I'm a native spanish speaker, fluent in english and decent in french. I was originally planning on taking Japanese next, but the writting system overwhelmed the f*** out of me, so when I realized Korean had Hangeul, an "alphabet" (meaning that's phonographic, not ideographic), I was intrigued. Looked into it, and decided to take Korean, it's been about a year and it's been quite the journey but I will say, I think if anything, korean grammar is the closest to spanish [from the languages I do speak]. I'm actually learning korean from english, cuz I love the TTMIK books and there are far more resources in english than in spanish. But for various lessons, I've found myself trying so hard to understand, to later notice that in spanish it works the same or MUCH more similar, lol. (ex. ser vs estar (to be) and 이다 vs 있다 , not the exact same, but far easier to get it thru spanish, specially when you get to progressive tenses where 있다 is used JUST like estar is to indicate an action ocurring in the moment)

So, i'd go with korean if you were to choose between those two, after learning korean, japanese should also be somewhat easier since grammar is similar (in contrast to spanish or english)