Can't comment on the 27 inch Dell, but I recently switched from a 24" U2410 to an Apple 27" and the main difference is the anti-glare. And it's a HUGE difference. The dell display had the "matte" LCD that you see on some laptops, no glare whatsoever. The Apple, on the other hand, is so shiny that you can see your reflection in it.
As for which is better, it's a matter of personal preference. I prefer the Apple display as the colors "pop" better for me, but I'm not a professional and don't know all the technical limitations. For me it just looks better, but you need to be prepared to deal with the glare.