I have the Dell Ultrasharp 24" and my friend has the LED ACD 24". I've spent quite some time with them and here's my opinion:
If you have a Macbook, you'd probably want the Apple display just because how it was built around it.
Both displays are 8-bit, the Dell is a S-PVA panel, and the Apple is a H-IPS panel. Viewing angles on both are superb, but you will get colorshifting (Red will look magenta) with the Dell if you look at it from a difficult angle. If you are in design, you could spring for the Ultrasharp, but generally designers want IPS panels because the slightly better image quality. Both monitors are far better than cheap TN panels.
The Ultrasharp is height adjustable and can pivot 90 degrees. I've never used it in portrait mode though.
The Ultrasharp also offers a plethora of connections if you want to hook up stuff and use PIP, split screen, etc.
The Apple display has built in speakers, isight camera, and a mic.
It's all up to you. I used to have a PC and my Mac hooked up to my Dell 2407, and it was easy to switch between the two. But now I just have a Macbook, so I wouldn't mind the Apple Display.