Larger and more importantly, significantly faster hard drive in the iMac. The iMac will always subjectively feel faster than the MacBook because of the faster hard drive and (to a more limited extent) the larger display.
So: Opening programs, any programs - advantage iMac
Ripping CDs/Compressing iMovies - advantage MacBook
Working in iMovie/iLife as a whole - iMac (due to the larger screen)
portability - MacBook
I have a macbook (2.16) and LOVE it, it has been my fav apple purchase to date. (Have had intel iMac, macbook pro, apple tv etc)
And I do mean LOVE it. I never thought I would get the use out of the portability as my travel usually is leass than 6-7 weeks a year and most of those Im traveling with a work win laptop. I end up pacing the work laptop and use my macbook when not "working".
That said, I sold my iMac a few months ago expecting the new iMac any day....finally...well...not exactly thrilled with the new ones - but after today's visit - growing on me.
I will not give up my macbook - it gies everywhere with me and spends as much time in the kitchen/family room/bedroom etc as my home office.
However, It simply is not as powerful and I want to stream stuff to my A TV and XBOX 360 and it becomes alot less portable when I have to have it hooked up to my externals drives. It takes longer to do most tasks (burn cd'd, dvd's encode h.264 - you name it than my iMac and I really miss that. I also am tired of spending most of my home time looking at a tiny little screen. In fact most of the features I LOVE about my macbook - become irritating when I am trying to get stuff doen on it in the office - or when I run out of space etc.
SO unless you really, really need/want the portability - suggest an iMac. I'm getting my new iMac (just not sure whther its the old or new one yet) and keeping my macbook. Hence the best of both worlds!
If I could only do one....God that would kill me....it would be the iMac.