How so? I would imagine the panels are the same if that's what you're asking. Your post title implies you're debating between a 27" Cinema Display and an iMac for your Mac Pro. You can use the iMac as an external display, yes, but you'd have an extra computer, so I don't understand the question.
It is actually the same. Same size, same resolution, same panels.
But it is not as high as the 27" iMac, when both (display and iMac) are positioned right beside each other, the display is a bit lower, they don't align.
I have read may posts in the iMac forum about black spots occuring over time on the iMac display, possibly from the close proximity of the GPU and power supply. The 27" cinema display would not have this issue. This and limited upgrade possibilities were the reason I ordered a MP.