I like the 27" personally, but it also depends on how close you are sitting to your screen. Currently I am using the 27" ASUS you listed there from Newegg. It's amazing, but I am currently wishing I got the smaller one. Or a new desk.
They both support the resolution you're seeking (1920x1080), they both have the same response time, they both support DVI so no quality difference, the 27 has a higher contrast ratio. I mean it's up to you, I'm positive you won't notice a difference in quality because they're so close in specs, the real question is, 27 vs 24. I recently upgraded by 2x 21' displays to 24' displays and wow, it's like a wall of screen. I have no idea what a 27 would be like, but 24 is large enough for me.
Both play at exactly the same resolution (1920x1080), HD6870 will look splendid on both.
I have never found anything larger than a 24" worth the massive prices you pay for it. Then again, I have a 42" HD LED TV for all my big-screen gaming requirements so I don't see much point with massive monitors.
But yeah, you can't compare the two because they're in two different price ranges.
If money isn't an issue, obviously go for the bigger and higher quality 27" (assuming you can sit a safe distance away from it).
23.6" - 24" is the sweet-spot for gaming for me.