I am Korean-American. I am going Korea this month and I will go to china in May. should I get a visa in U.S.?

or in Korea? My status in Korea is F-4 and it is valid 12/12/2014. If I can get a Chinese visa in Korea, I want to get it in Korea. I do not have time here to get it. and I also want to know what kind I can get and how much money I need it for? however I need one for travel and i want the longest one.