I'd only got multi-screen gaming with 3 or more anyway, and of the same size and resolution. Sup Com is about it for dual screen games that are cool with different screens.
So you can still have both screens, just run one with the game and the other with whatever when gaming (or not at all). Dual screens is more a productivity thing than a gaming thing.
Anyway, both monitors are 5ms, the 2ms(gtg) is marketing bullshit to make it look better. GTG = Grey to Grey. So it's not 2ms unless you play grey games. Most games tend to be quite deep blacks and contrasting colours. (It's like that whole "100,000:1 Dynamic contrast" shite. Means fuck all in real life once you toss in "dynamic")
So I'd go with the bigger monitor, especially for gaming. They're the same resolution, so you're gonna have a lower DPI on the bigger screen, but I've got a 27" 1080p screen and it's not like it's a pixelated mess.
So yeah, get the $180 one, and use both screens. At least give it a spin for a week or so. Especially if you're on Windows 7.