Apple ACD is the worst thing ever made by Apple.
Just google "Cinema Display Defects" and you will have millions of results. I've bought one earlier this year and returned it after two replacements. (Light leaks, dead pixels etc.)
Dell makes ugly computers, but they are large WS LCDs are the best in terms of quality and value.
With DELL you will have TONES of extras as a high end monitor, even including Component ports that allows you to connect to your Digital set-top box (and an instant HDTV) directly. You also got USB hub, card reader, rotation abilities etc. Don't worry about Dell's "Banding Issues" in case you found it somewhere on the net, it has been all solved by a firmware update.
ACD? Despite the fact it looks good (when you put it next to mp or mbp, for the match), its a decent peace of expensive ****.
If people here saying ACD has better image quality, don't bother. They have been saying this for ages, even when Apple were actually using the same LCD panel (philips) as Dell's. Afterall, if u really want to go pro, u end up with Eizo or Wacom (lol).