Well the biggest question here is what are your specs? It's going to take a very powerful computer to push 120 fps at 2560x1440. Assuming you have a computer powerful enough and the extra money to spend on the monitor then the Asus should be great.
Ive just about finished my build it's pretty decent, 4770k 16gb tridents vi hero etc. using a 780ti or tri-x 290x. The thing is you can oc said korean monitor to near 120 hz with 1440p and it's pls. The fact it's tn is what's putting me off.
Overclocking those monitors is not the same as the ASUS monitor. Also, you have to buy one of the tempest (I think that's the name) monitor which is "certified" to overclock to 120 Hz. If you can afford it I would definitely suggest the Asus.
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.