i like the 27.. because well, its a 27.. you seem pretty decided so thats my vote, but have you checked out some of the new IPS panels that just hit shelves in like.. Dec 12? there are a few of them all based on LG panels. Theres a viewsonic my top pick was the AOC I2757FH. the reviews on it all crushed, and its only 300$ for a sick ips.
its not 3d ready though, so if thats your bag id still say the 27.
spec wise the 24 takes it, but my such a low amount i really don't see it being noticable. So basically do you want to spend more for the size or not. the differances in response and such shouldnt be noticable so id say it boils down to budget and size preference.
if you have played on a 27.. ever, you will not want to go back, just saying
I should note that i have had a 2560x1440 27 and a 1080 27, i think games still look awesome at 1080 and wouldnt consider the game being "stretched" to that size an issue.