Normally I am nothing more than a lurker on these forums but thought I would jump in and make a quick post here.
I just recently set a Mac network for a client, we purchased 2 24inch refrubs (the same you refereed too), a 21.5 inch iMac and the base 27 inch iMac.
As far as performance I didn't see a huge difference, except the fact that the new iMacs can handle up to 16 GBs of RAM.
The one area I noticed the most and had many people I work with comment on was the color difference between the two. The 24 inch iMacs looked washed out when put next to the new LED 21.5/27 iMacs. Alone they looked great but when compared to the new ones it was amazing to see the difference. If you are going to be doing design work then I would want something that would show the colors as true as they could, which in my mind would be the new ones.