I think, from what has been said in other threads, there may not be a big difference in the performance of C2D and Core i CPUs, the difference maybe in power consumption. I haven't looked at iMac spec/prices so not sure how these compare to MBPs.
I have a 24" iMac with a 2.8GHz Dual Core C2D & 4GB of RAM.
I built a Hackintosh with a 2.66GHz Quad Core i5 & 4GB of RAM.
The i5 absolutely smokes the C2D.
When using the CS4 suite, its performance is amazing, which I don't know why they don't use them as the base processor in the iMac. It's an option, and a damn good option to get too. Same with video encoding, the i5 just flies. I can't wait for CS5 to fully take advantage of OS X's power, I can only imagine it will be even better.