It also comes down to budget. A quality 2560x1440 monitor will cost more.
I absolutely love my Dell U2711 2560x1440 monitor, though I might not buy the same one today. You must research these CAREFULLY as many have issues. If you had an NVidia graphics card I'd suggest buying a G-Sync monitor which runs very smooth with minimal lag at even 40FPS but you don't. While I play most games at 1920x1080 since they look basically IDENTICAL to higher resolutions (like Skyrim) some games greatly benefit from 2560x1440. mostly top-down games with small text/HUD elements like:
- Starcraft 2
- Diablo 3 (if I played it)
- Torchlight
- CIV5
I find 60FPS to be pretty smooth for most games. In fact, the problem with a 60FPS game which is NOT smooth is usually not due to the frame rate specifically but an issue with stuttering, and pop-in.
I went back and played HALF LIFE 1 at 60FPS with VSYNC ON and it was far smoother than other modern games at 60FPS. I'm not saying 120FPS can't be smoother, just that I don't need it to be better than 60FPS.
*It's also important to note that you need to generate above 120FPS in a game if you wish to synch to 120FPS VSYNC otherwise it resynchs to 60FPS anyway, and I don't like VSYNC OFF due to screen tearing (which gets worse at higher frame rates, not better).
It's VERY difficult to get above 120FPS consistently in a modern game, unless you sacrifice some visual quality which to me isn't a good trade-off.