This is actually a very good question.
You honestly have to weigh up what you're after and what you're willing to compensate for.
--With a Korean monitor, you're receiving a higher quality display (in terms of eye-candy), unfortunately in comparison to a TN panel, your refresh rates are going to be slower (about 5/6ms (advertised)), vs TN panels going down to 1ms (advertised).
--You're also going to have to think about whether you will be getting your 120Hz at 2560x1440. That's a very high frame rate you're asking for, even on top of the range hardware. In terms of expense, can you afford to always have the top hardware in order to always maximise your monitor's frequency.
--Warranty. This is a biggie, buying from the Korean markets can be excellent value for money, but you're sacrificing British/American warranty to do it. IF your monitor goes faulty you're looking at quite an expense to send it back (I believe it's around £80). As a side point to this you will also have to weigh your options as you may receive a dud or one with a few dead pixels.
Monitors are a lottery game just the same as CPU's.
My experience if you can be bothered to read:
As a way of assistance, I personally upgraded from a ViewSonic 24 inch monitor to a 27 inch Crossover (another Korean branded one) that's 2560x1440 S-IPS (there wasn't many of the newer AH-IPS ones).
In terms of the service I received, the monitor was well packaged and delivered quickly. I found various faults with mine however:
There were about 4/5 dead pixels on arrival and the casing wasn't properly in-line the glass.
After contacting them and telling them it arrived like this on arrival, they were able to slice 20% off the price I paid, knocking it down to around 180 from 220 something. Maybe I was unlucky in some sense, or maybe I was lucky.
Choose wisely on your monitor though. Thanks.