I use the Apple 20" at work. Very nice monitor. I recently purchased the Dell 20" for my home. I wanted the Apple one, but I figured I'd save some money this way. I do light graphics work on both and I cannot tell the difference is when looking at the screen. Here are my advantages of both (in no particular order)
Apple:
1. Looks. It looks so much better than the Dell. 'nough said.
2. Closed case. The case does not require vents to allow heat to escape like the dell has. This keeps dust out and might even extend the life of the monitor. I doubt I'll keep it that long though.
3. Firewire. The Dell does not have firewire ports on it.
Dell:
1. Maneuverable. This is nice. Up and down. Left and right. Flip it 90 degrees. This comes in handy and is my favorite feature.
2. Connections. I have my laptop connected to DVI. I have my desktop connected to VGA. I have my PS2 connected to Composite Video. I have my TiVo connected to S-Video. Much better than Apple's, but this is why the Dell needs the vents for cooling.
3. Split screen and Picture in Picture. I watch TV and work on my computer at the same time. The screen can get small, but its useful at times.
4. Price. Its just a better value.
5. Versa mounts. I don't use it, but you could if you want to.
If I had to do it again, I'd choose the Dell hands down (never thought I'd say that

). I know I'm comparing the 20" to the 20", but I hope this helps anyway.