Sandy Bridge CPUs are fast, but they're not 40% faster per-clock, which is what would be required to catch the i7 iMac.
Personally, I prefer the greatest screen real estate. That's why I have the quad i5 iMac (it's a little slower, no hyper-threading and only 2.8GHz) and a late 2008 Macbook Unibody with a second hard drive in the optical bay for mobile recording. I can track on the slower mobile machine and then mix on the big screen with extra power. Truly the best of both worlds.
