It's a case of "Once you go black you can't go back".. What I mean is, once you game on a 120hz 27" monitor for a few hours, and then go back into 60hz 24".. You will never feel the same again. The screen just looks weird to you, even if it was the same size, something is off. The FPS. It's not the same as switching between 1440p and 1080p for example, it's the smoothness.. it plays with your brain, as it is not just the image quality.
I am avoiding gaming at 120fps in all instances, not buying a 120hz screen etc Because I don't want to get used to it before I can afford the hardware for it. I rather settle for perfect 60fps than play games that "Could run better in my eyes but I don't have enough grunt".
I love Vsync by the way and I don't notice any input lag, even though my Vsync keeps flipping on and off on the fly (adaptive Vsync).