I have actually compared them both. I thought the picture quality between the two was similar. The Apple is much bigger so the pixel size is a little more noticable. I had problems with the features on the apple, or lack-there-of:
-only has the one Attached cable coming out of it that leads to a break out box for power and such
-limited features, like usb, card readers, pip--they focused on minimal user interaction which I don't like.
-no other inputs, component inputs on this should be obvious..would make a very good 1080p monitor.
-we had trouble with it on non-dual link DVI cards and unix machines
For this reason we only buy them for rare, highly specialized needs. They are impressive initially but our productivity seems higher on the 24. The 24 is becoming our mainline monitor due to its performance, cost and ability to work with all our hardware.
If you have the money and the compatible hardware the apple is nice but does not beat the dell in a cost based analysis.