Of course Apple! Always pick Apple! Apple makes hands-down the best displays ever! Dell, on the other hand, makes the crappiest displays in the world -- Steve Jobs himself even said "they buy the panels that we reject!" Get the Apple -- at ANY price! Apple is like God and Dell is... well, Dell is crap!
... just kidding, of course.

I haven't checked, but in all likelihood it IS the same panel underneath the plastic or metal enclosure. (Edit: oops, right, 23" and 24" obviously not the same exact panel. But probably the same factory...)
I use a 20" Dell at home (well, I did until I got the 20" iMac) and if I was looking at a 24" panel the Dell 2407 is definitely on my list of screens to check out.
What it boils down to, assuming you calibrate both panels correctly, is aesthetic appearance. Personally I'm not willing to pay almost twice the price just for a fashion statement. Keep in mind the Dell includes a selection of video inputs too, including s-video and composite, with Picture-in-picture functions and whatnot. If that's useful to you...