120 is just a frequency of a monitor, just like 60, 75 and so on. Some people have issues looking at different frequencies, giving them subliminal headaches. >.< something to do with your eye trying to keep up with the screen changes. Some monitors produce a crap picture at either 120, 100, 75, 60, depending on their quality and I would imagine the gear they're targeted at. Many newer office based will be 60/75, older ones can do some very strange numbers. You can pickup flickering text, others can't. Blurred text is a strange one. 60 I like myself. Figured the 120 ones just did 2x60 for 3d. Not sure if I could suffer a 100 screen for long even tho they look decent enough. Takes me a while to adapt to different monitors/frequencies/sizes.
fps is very debateable. 25/50 should be at a point where the eye will notice. All vsync related tho. People like 50/60 cos they figure there's a cat in hells chance the monitor is missing anything.