If you can't decide, I would say just learn both.

As you progress, you'll be more exposed to the languages and their culture, and one of them will come up as the winner.

And whatever you learned about the other language is your asset too.  I doubt anything you learn will ever go to waste.

There is practically nothing similar between French and Korean, so learning both in parallel won't confuse you in any way.

My choice would be French and the reasons abound:

What do we know about Korean culture and its cultural achievements? They invented the Tae-Kwon-Do, they are one of the "Asian Tigers", they have two major competitors in the economic global arena (Samsung and Hyundai) and they eat dog and horse meat. :-(

Apart that, their language is completely alien to a westerner. You can't count on your previous knowledge of the languages you already know. To make things worse, you'´ll have to learn a completly new set of symbols because they have their private alphabet.

A language is, above all, a tool that we employ to communicate, to learn and expand our vision of the world. Once you have learned Korean, who will be the persons you will talk to using it? Where are they located? What opportunities you will have to use (and maintain) this new knowledge?

On the other hand: French is the lingua franca of diplomacy. It is spoken in Canadá, Belgium, France, Switzerland and a lot of countries in Africa, Pacific and Caribe.

It is a romance language, not too hard to learn for someone who already knows Spanish.

France is one of the most advanced economies of the modern world. It has lots of history and a huge tradition. They invented "la dance", "la gastronomie", "haute couture", among others. They have a very strong literature. Louis XIV, Napoléon, Victor Hugo, Debussis, Rodin, Dirac, Camus, were all French (Can you name a single Korean universal personality?)  

Of course, each people has its own culture and history. And each one of them has its own value. However, the volume of realizations made by the French civilization has no parallel in modern world. And their language is the way to access all of that, without brokers, "in natura", first hand.

And there is also Paris!

If I were you, my dear friend, "je voudrais parlez français!"

No doubt about it.

;-) 

Tony has a good point there. I would say just do both. You may progress and find you love both or may just end up loving one. 

If you don't want to start both at the same time, then give one a week and then the next a week. After that, you should have a better idea about how you feel about both languages. 

Mayumy, you're well on your way to speaking a lot of languages. And if you ever meet other polyglots, you'll probably enjoy conversing with them in several languages that you have in common with them. It seems that almost every polyglot learns French, Spanish, and/or German. If you know at least two of these languages, you will be able to converse in more languages with more polyglots.

I've also heard that some of the best language courses were not created in English, but were created in French, German, and Italian. So knowing any of these three languages could make it easier for you to learn any other language you want.

But it primarily depends on which one you'll be able to use more after learning it. Even French can be useless if you have no one to speak it with. So that depends on your situation and interests (as other commenters have said).

Although you'll learn French a lot faster, the difficulty of a language is not so important. If you are passionate about a language, you'll persevere until it's less difficult. If you have a strong inner desire to learn Korean but not French, you'll quit French after a while but you'll always come back to Korean. Or vice versa. "Everything is hard before it is easy."--Goethe. Even Korean will become easy eventually. Personally, I find spoken Mandarin Chinese harder than Korean--but even French used to be difficult sometimes.

Considering you already know Spanish, French will be much, much easier to pick up for you. They are both Romance languages so they have a lot in common, be it vocabulary or grammar.

Korean is very hard and will take you exponentially longer to learn. Its grammar is not even remotely similar to Chinese (actually, your knowledge of Chinese won't help you with learning Korean at all afaik). The sentence structure is completely different (subject-object-verb) and Korean is also highly context-sensitive which means that it regularly drops the subject of the sentence. That can be especially hard to get used to at first. 

The grammar is probably also the hardest aspect of the language. I'm by no means an expert, but anyone who has tried to learn Korean can attest to its difficulty. For starters, it has five levels of politeness that change the predicate and nouns in the sentence. It also has a case system which you have to learn.

I'm sure someone with a more advanced understanding of the language can give you better insight as to why Korean is so hard to learn, but even if you don't know all the details, the one thing I want you to take away from my little write-up is that learning this language won't be a walk in the park. It'll probably take you many years more to become proficient in Korean than in French.

On the flipside, Korean's writing system is very easy to pick up, and I personally think it's a very neat language and worthy of study. Whether you still want to take a gander at it is up to you.