I faced the same dilema 2 months ago, and finally decided to go with the high end macbook Pro plus a 27" monitor. That way at the office i use a dual screen setup with both the built in monitor and the 27" one. My main work is done on the 27", but i have email and icq open in the built in 15" monitor.
Even if you don't travel a lot, the portability is nice to have. I don't travel a lot either, but instead of having to sync two computers at the office and home, i just carry my macbook pro. Right now i'm in at my wife's family home using the macbook pro, something you can't do with the imac.
About the Ghz difference, have on mind you are not talking about same processors. The Imac uses i5 while the macbook pro uses i7 processors with Hyperthreading. Some tests shows that performance is around the same with some advantages for one or the other depending on the applications used. Check here:
http://www.macworld.com/article/1596...enchmarks.html
In resume, if you NEVER travel or NEVER use the computer outside your office, go for the Imac, if you travel from time to time, or move from home to the office and want to have access to your data at both places, or need to have your data available while working on the couch or checking stats at night from home or on weekends, go for the MacBook Pro.
In your situation I would discard the macbook air, as those are for max portability but have less power than the pros or imacs.