I know that both the iMac and the MBP would be well equipt to handle what I would use it for. Portibility is something that would be nice, but it's not something I would use everyday. I quite a clutz, so going to a coffee shop with a nice new laptop is a no-no for me. I have a G5 iMac right now and absolutely love it. Err, lovED it. It has just joined the ranks of all the other mysterious randomly dying iMacs. I know it's fixable, but I was going to get a new computer soon anyways.
I'm really looking for lots of screen space, or as I once heard it: "real estate". 50% of the time I use the computer for everyday things, and the other 50% of the time I deal with very large photoshop files and video files, with the occassional game in there also. I also plan on getting Starcraft III and Diablo III when they come out.
I'm still undecided cause both computers can handle all of that.