I asked myself the same question a few weeks ago while shopping for my first Mac. I went with the PB because it was more upgradeable (I know that sounds weird, but I'll explain). If you're one to change computers every 2 years or so, the iMac has it's monitor built in, so if you sell it, you'll have to get a whole new monitor unless you get another iMac. If you're in college, the 12" PB would probably be perfect as it's VERY portable for class. I got my PB because I plan on getting an ACD soon (either a 20" or 23") to hook it up to; then, when I go to college next year, I'm going to buy a PowerMac to hook up to the monitor (I already have keyboard/mouse) and keep the PB for portable stuff.
If you think you're only going to have one computer for the next few years, the iMac is a heck of a deal -- very powerful. But the PB is a very capable machine for future uese (just max out the RAM; I haven't done mine yet and I get the beachball a lot, but only very briefly).
Good luck! I am sure you will be happy with whatever you choose
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