Hm, it's not really an Apples-to-Apples comparison, b/c all the 27" 2560px displays I can think of are IPS, while 1080p's in the sub-$300 range are TN's. You'd be paying for the added display quality on top of the bigger screen + resolution.

For dev work, I'd say the nicest thing about the 27"s is the ability to put two full pages side-by-side, either a browser window + IDE, or vert. split IDE code panes. 1920px isn't quite there for two panes of code.

And personally, I've had 2+ 24"s, and I've actually found it too cumbersome: having emails/IM's on a second window actually becomes distracting, and moving your mouse cursor across so many pixels was less precise than either alt-tabbing to the right program or switching spaces/virtual desktops w/ a keyboard shortcut. For me anyway, 27" is the sweet spot.