A few thoughts...
-A single 30 will allow you to view more image at full zoom or see an entire image with more magnification than a 24.
-While two screens would be nice I think its more important to put the emphasis on the image itself rather than the palettes; its always nicer to have a larger photo/canvas when working, the amount of space palettes need is trivial in comparison.
-A single 30 has 4,096,000 pixels vs. 4,608,000 pixels of two 24s so you're not really gaining that much more screen real estate and youre probably not going to fully utilize the second 24 for palettes anyways.
-Both photoshop and lightroom menus nicely tuck themselves along the sides of the screen and leave plenty of room for the photo, I dont think you'll feel that you're in need of more space with 1600 pixels of vertical resolution.
-I dont know about the quality of the panels, but that aside, a 27 isnt going to get you any more screen real estate (pixels) against a 24, so assuming colour quality is equal, its a matter of dpi and the 27 being easier on the eyes than the 24.
-You may want to check up on this, but I think the IPS panel is becoming less common in todays 24s (unless you start looking at the pricey nec/eizo options) so you might end up with better colour on a 30 as well.
-Ive used my 30 cinema display in conjunction with my 15 macbook pro screen and a wacom cintiq; Ive never really felt the need to use the dual screen setup for additional palette space. Ive also used an older Dell 24 at work (for graphics as well) and I definitely preferred the single large screen to two smaller ones although in my case it was the 24 + a 19.
I hope this helps a bit
