Whether you see difference between 60 fps and 120 fps is mostly personal. Normally i'd say you will need to get your fps up to those levels first but since you have 3x gtx680, i'm going to assume that's not a problem
Some people cant even see the difference between 30 and 40, some people are sensitive enough to notice the difference between 100 and 120. Which one you are, no idea. You need to decide that for yourself.
If you are in the latter group, it makes sense to go for the 120 Hz screen, otherwise i'd say not really.
The difference between 1920x1200 and 2560x1440 should be clear though, higher resolution on the same size surface makes for a sharper image by definition (though text and such is also smaller).
One thing you should consider though are your videocards.
If you have the 4 GB version you should be fine for the higher resolution, but if you only have the 2 GB version you might run into your VRAM limit on the higher resolution screens.