Two cases:
Seoul
Pros:
- Center of everything, including administration and police.
Cons:
- One of the highest population densities in the world.
- The only river, which flows straight through the middle of the city, is heavily polluted.
- There is not enough food in Seoul to feed everyone for even a day,
- Seoul is surrounded by lots of satellite cities = even less food
So... if it's just Seoul, we're screwed.
South Korea
Pros:
- Plenty of technology.
- Has some good steel mills.
- one hundred percent (male) military training xD
- Little wildlife in the way of wolves or bears. Mostly boars, deer, and tiny animals.
- "Korean will" as purportedly evidenced by the history of repeated invasions and the removal of the invaders, and the subsequent rebuilding.
Cons:
- 27% self-sufficient, rice-wise.
- Nowhere near enough arable land to feed everyone.
- No oil.
- Little naturally drinkable water. Sure, filtering is possible, but without oil? Hrm...
- Historically, there has been a period called the "barley pass", a time between the spring barley crop and after last year's rice has been all used. It tended to be when lots of people are killed, so yeah.
- Irregular rain and drought = hard to farm.
- Dependent on exports to make the economy go 'round, so the economical state will be chaos.
- Not a lot of wood in the forests of Korea, mostly due to the small area.
Both Koreas
Pros : Same as above, plus:
- Iron (even if low-grade) in the north
Cons:
- OH SNAP THE NORTH KOREANS AND THEIR PERM-HAIRED LEADER
Verdict:
Only Seoul? Cue everyone dying.
Only South Korea? We might get along.
Both Koreas? Screwed.