Better to buy it in the US, even if it would work.
1. Even though I believe the Korean phone would work in the US, you can pay the full price for the phone at the Apple Store in the US and it will still be a hundred dollars cheaper than the price of the phone with a contract in Korea.
2. I recently bought an unlocked iphone 5 (verizon) from the Apple Store in the US and brought it to Korea, where I will be living for the next few months -- I travel a lot and needed a way to pop in local sim cards. Everything worked fine for two weeks, but now I have a hardware problem with the phone and have no way to get it conveniently replaced.
After contacting Apple and Apple Korea about the issue, I was told to take the phone to a trusted service provider here in Korea to see about getting it replaced, albeit a Korean model, as they sometimes offer replacement phones to travelers in need of an immediate replacement. The phone would still meet my needs so I agreed. Upon arriving at the service provider, I was turned away with little more than, "Nothing we can do," although fully recognizing that there was a hardware issue.
A tad peeved, I immediately called Apple Korea, told them the issue I was having, and was instructed to hand the phone over to the engineer so they could speak. Less than two minutes passed, he hung up, and I am still in need of a replacement.