As a Chinese, I'm willing to remind you that learning Chinese is not that rewarding as learning IndoEuropean languages.
Agreed. It takes 5 years to bother speaking Chinese fluently and 10 to learn Hanji.
So Spanish will be the most "useful" language to learn.
Seconded. Out of all the languages Theymos mentioned, I think Spanish is the most useful and rewarding.
Learning ANY second language makes it easier to learn ANY third language. It wires up the brain for multiple languages. So don't worry about that.
At least when that language is in the same family of course. Learning Korean doesn't help me learn Vietnamese any easier at all. Learning English though sure does help me learn Spanish!
If you're in the US, Spanish, absolutely. You'll get to use it all the time.
Agreed. Languages are learned to use. Spanish is actually useful, although not very exotic in the USA. Too bad Korean is not on your list.
Whatever you do, stick to it.
Couldn't have said it better myself. I have never studied Korean once, but I'm fluent in reading, writing, speaking and listening, only because I stopped using English when I got here and wrapped my life around the process of using it continuously. The result? My English got worse every day, proving that continued usage is key.