It's personal preference, but I'll give you some points:
1) I prefer my LARGE 27" screen, even though I game at 1920x1080 most of the time because most games at 2560x1440 look almost exactly THE SAME. Main exceptions are Torchlight, Diablo 3, Starcraft 2 etc (games with small text/HUDs). Even at 1920x1080, it still looks better than a 27" with a NATIVE resolution of 1920x1080.
2) I prefer 2560x1440 for desktop applications (must setup right: DPI scaling of 25% or so, and CTRL-Mouse Scroll to change size in browsers).
3) Some 120Hz monitors have a 60Hz HDMI input (DVI is 120Hz PC format; HDMI is HDTV format).
4) 3D support?
5) Even a GTX780 will struggle to maintain 120FPS in many games, though you still have the option of Adaptive HALF VSYNC for 60FPS VSYNC on 120Hz monitors.
6) *MOST 27", 2560x1440 monitors have issues such as bleeding etc. Be very careful when buying one. The one with the least issues may be the Dell U2711 but it's also $1000. Other than slightly sparkly anti-glare (doesn't affect gameplay) mine's been perfect for three years except when it failed but the new monitor arrived next day with a pre-paid waybill so I paid nothing.
SUMMARY:
If you can find a 27", 2560x1440 screen with no issues, it really is awesome to game that large. The large screen makes you feel more immersed and it's great for top-down games like Starcraft 2 with the extra real estate.
Be prepared to do a lot of research and spend some money though.