You're actually comparing two different types of monitors which were built for two different purposes. The BenQ XL2720T and the Asus VG278HE are built primarily for 3d purposes. The 120/144hz is just a great byproduct feature. The QNIX is one of the many Korean hi-res monitors which are NOT 3d. If you are looking for extreme gaming quality and possible gaming in 3d, either the BenQ or Asus are choice monitors. The Qnix along with many other Korean monitors like X-star, Yamakasi, Crossover, and many more are 27" 2560x1440 monitors that are cheap alternatives to the expensive 2560x1440 available in the US. Those Korean monitors (Qnix, X-star) are PLS monitors using A/A- grade panels from Samsung, and the other Korean monitors (Crossover and Yamakasi) are IPS monitors, again using A/A- grade panels from LG. The prices on the PLS range from 260-350 and the ips 290-450ish. They are great monitors to use for everyday purposes.
If you are going to use a Korean monitor for gaming, I recommend the PLS monitors because they have faster response times, and the Qnix is capable to be overclocked near 100-120hz. These monitors will stress the memory on the video card (high res), so it also depends on what kind of video card you have for gaming. I have a gtx 670 4gb, and I've seen my card easily exceed 2gb.
For light gaming and everyday purposes I'd recommend the Korean alternative monitors. Purchase them from reputable ebay sellers. The only drawback is they have really crappy frames (plastic), but the visuals are not affected.
A monitor for pure high performance gaming and 3d gaming, you need to get the BenQ or Asus model.
Extra Note:
TN - gaming benq/asus, extremely fast response times 1-2ms
ips - great visuals and wide viewing angles - LG panels
pls - korean monitors use this as ips alternative (panels made by Samsung). Many people argue it is slightly better than ips. Almost no loss in visual, but faster response times.
(edit: I just saw your second post about your radeon 1gb card. 1GB cards were great in the past, however games nowadays with even 1920x1080 res monitors can use 1.5-2gb ish. I have an X-star 27" and it goes well over 2gb at times. If you're on a tight budget I'd recommend upgrading your card to something with 3-6gb (whatever is affordable to you) and then maybe buy an x-star or Qnix.)