You can lug an iMac between houses. If you need to go places with it, no question get the Macbook.
The MB is faster, I think. More memory, faster memory, not sure about the processors though.
The iMac has a nice big screen though, and firewire.
I know they're both 2.4 GHZ, I'd suspect a different front side bus though.
Anywho, the MB is also more expensive by a couple hundred. $1600 vs. $1200, I think?