English versus Chinese
Short answer is English.
Yes, 1.39 billion people speak Chinese, and only 527 million people speak English. However, if you consider the number of people who learn English and the number of countries in which English is spoken, English wins hands down. 1.5 billion people learn English versus only 30 million learning Chinese. English is regularly spoken in 101 countries, while 33 countries do this with Chinese. The Chinese make up a large portion of the 1.5 billion people who learn English. As such, if you speak English, it is likely that you will be able to communicate with Chinese people as well.
While the ubiquity of English can find its roots in England’s imperial past and United State’s global hegemony over the past century, its influence is much deeper now than ever before, especially through social media and pop culture, and most likely to remain so for the foreseeable future. English is the language of diplomacy, law, trade, finance, academia, science, and computing. If you speak English, you can work anywhere in the world and, in the least, have more opportunities than peers who do not speak.
No doubt that China is a great opportunity for Korea. However, it will take China a long time before it can catch up to the English speaking world in terms of core science and technology, original content, rule of law, and legitimate global leadership. While it has a rich, ancient history, modern China is barely 40 years old.