Why not both? Overlord currently has 120hz 27" 1440p monitors up for pre-order (shipping late this month)
If budget was an issue - then i would probably say 24" 1080p 120hz if youre any type of gamer, heres why (keep in mind my 120hz monitor is still on the way
120hz TN 24" Pros:
- If youre sitting dead center of the monitor - it still looks decent - color shift wont be too bad
- color accuracy is NOT terrible on new TNs, yes IPS looks sharper but its not like its night and day
- vsync on introduces display lag, sure you wont get tearing but its not uncommon for people to feel the lag
- vsync off you get better response, but then you get really annoying screen tearing
- even the fastest IPS panels cant tough the response time on TNs right now (5ms vs 2ms or less) so youll get absolutely 0 ghosting. On my 6ms CrossOver, i dont really notice it to be honest but some people might
- GPU requirements to drive 120fps @ 1080p isnt bad as 1440p for obvious reason, for even more performance you can cut AA
- I suppose you can do 3D if you like
1440p IPS 27" Pros:
- biggest reason - lots of real estate - high resolution, things look sharper
- It looks plain better, from all angles - you can have a nerd slouch and the display will look amazing
- Color accuracy is better, again not magnitudes better - but definately noticeable - especially if you have a screen with no anti glare
- at 27" and 1440p, the pixel density is so good that you really dont need AA at all for a big performance boost - i only use it when im already pushing past 80fps
- ability to go portrait - again no color shift, some sites/formats look better in portrait mode - wanna read a comic book? Looks AMAZING when the full page is displayed
- everything just looks better. Not just games, but everyday use browsing, photos, videos/movies, etc.
One more note about IPS panels, is this thing called IPS Glow - at first i thought it was backlight bleed but it isnt - which kind of forces you to sit a bit far from the display (about a foot and a half to 2 feet).
Anyways hope this helps, myself - i cant wait till my 1440p 120hz comes in.