Speaking from direct experience (not something I read in a forum post on the internet), I'd recommend 120 Hz in a heartbeat.....tho more on that later I have two setups almost side by side up stairs.
One has a GTX 580 w/ Dell IPS Panel .... Son No. 2 is minoring in Photography at college and IPS made sense for this application. A+ in Photography ..... between B+ and A- in gaming
Son No. 3 has twin 560 Ti's w/ Asus 120 Hz TN panel. While it can't touch the IPS panel in true color rendering for photo apps, Son No. 3's box blows away the one above in gaming.....fps, game image quality, brightness, response time are all well above the Dell IPS panel which frankly looks "washed out" in games by comparison.
As for 3D, frankly 3D movies don't do anything for me.....and tho I haven't watched one on this PC, Son No. 3 bought himself the nVidia 3D vision glasses and I did play Batman 3D and parts of a few other games. It was a kick and surprised the hell outta me.....I enjoyed it immensely . To the point where if a game had 3D capability, I couldn't imagine not turning it on.
As to the size, worth doing a bit of research on dot pitch and pixels per inch. The human eye (normal vision) can start to distinguish individual pixels at about 96 ppi at normal desktop viewing distances.
http://en.wikipedia.org/wiki/Dot_pitch
Here's some common PPI
1920×1080 23.0 95.8
1920×1080 23.6 93.3
1920×1080 24.0 91.8
1
920×1080 24.6 89.6
1920×1080 27.0 81.6
1920×1200 23.0 98.4
1920×1200 24.0 94.0
1920×1200 27.0 83.8
2560×1440 27.0 108.8
2560×1600 30.0 101.6
So if ya it at normal desktop viewing distances, the ones in bold will likely appear (to most people) a bit "grainy".