This statement suggests you're unfamiliar with modern cpus. Turbo boost has limitations. It's not just 2.9ghz for a sustained period of time at will. The imac is obviously clocked higher but with an older architecture. You can't just pick out whichever is larger.
I'm surprised you want to go from a 27" display to an 11". Have you spent much time with a small laptop display? Did you require portability? Personally I'd suggest the 13" especially as it bumps you to 4GB of ram which is the least i'd suggest specifically under Lion. I know I'll get comments that the ssd negates this, but ram is still superior, and the 13" display is much easier to look at for extended periods of time in my opinion.
I would use the Air with an Secondary Monitor which I would use if I'm at home, so the 11" would be no problem for me.