Jason N. has taught in both Korea and China. He's enjoyed both countries and has written a great overview on why China may be a great choice for anyone looking for a change.
Both countries are terrific, but I'm incredibly biased and prefer China by a country mile. Why don't we compare the lowest likely salaries for both countries? It would be around 8000 RMB plus housing as the basement for China and 2 million won on the low end in Korea. It's around $1,400 compared to $2,000 in favor of Korea. Unfortunately, that's where most people stop comparing. You will spend 40% less on food, and around 70% less on utilities, which would knock the benefit of the Korean salary down by quite a bit. I spend about $500 a month on food and bills, and I eat out pretty much every day. Let's add another hundred for unforseen expenses, and you would still end up saving $800 a month. In Korea, I saved more like $600 a month because everything is more expensive, and that was on a 2.3 million salary. Keep in mind that the teacher in China is teaching around half the classes as someone in a hagwon.
However, this bare bones comparison doesn't really do the Chinese market justice since you can make a killing on privates here, which you can't in Korea. I make 5,000 RMB a month on 4 privates a week. If a brand new teacher did the same, they would make the equivalent of 2 million won in a country that is half as expensive while teaching 12 fewer classes a week. You could save $1,500 a month if you don't travel around a lot. If you do travel, you could still probably average over a thousand.
Also, consider the location: Shenzhen is on the border of Hong Kong and a 30 minute train ride from Guangzhou. 50,000 people live in just these 3 cities, in other words, the entire population of South Korea. Shenzhen is also a new financial hub, so you will meet Chinese people from every province because they come here for work. You get a really well rounded sense of the culture because of this. Hong Kong kind of feels like another world, and I actually prefer it to Seoul since it's grittier and has a sort of New York vibe to it. It's even more international, and it's much more picturesque. The nightlife is also great there.
You'll enjoy either country, but it's hard, once you understand the life of a teacher, to justify teaching twice as much for the same or worse financial return and likely in a city that isn't as cool as Shenzhen since the Seoul market is saturated.
I hope this gives you my perspective. I actually switched from Korea after I kept running into foreign teachers who teach in China but were vacationing in Seoul. Discussions almost always led to them sighing in relief that they didn't have to work as much as me. I can't tell you how nice it is to be on the other side of that conversation. However, I totally see the attraction to Korea. The culture, language, and food are fantastic. You just have to ask yourself if it's 15 extra classes a week fantastic.