iMac - stronger video card, faster processor, more upgrades available.
MacBook - totally portable.
If you are looking for an every-day use computer, either one will work: the iMac just ties you to your desk while you can take the MacBook anywhere.
If you are going to be doing a lot of processor-heavy tasks or 3D modeling, get the iMac: the juiced-up processor and video card will be worth it.
I got my first Apple (an iMac) last September and I love it! However I'm going to get a MacBook (Black) really soon so I can take my Mac with me.