120HZ: Amazing in FPS games. Only 'works' if you give it more than 60fps from your GPU, which with a GTX670 you'll be pushing it in the latest games..
Games look really fluid on 120Hz and are more responsive to mouse movement. They are also only 24inch 1920x1080 resolution.
IPS: They look really pretty, have amazing colors, have wide viewing angles. Usually are limited to 60Hz (Yamakasi Catleap OC ones go higher), have a lot wider variety of recolutions and sizes.
No you do not need to run 120 fps to notice the difference on 120hz lcds. I have one and can vouch for this.
Hmm, I thought you had to. Is it just >60Hz to notice a difference? Even 61Hz lol?
..I highly doubt the human eye can see a 1fps difference ... 5-10 and higher increments, sure. But 1 is just too minuscule.