If you need portability, a MBP + an external moitor would be the best. If not then I'd recommend the iMac unless you really don't like the idea of a built in display.
Well put. Also a little more info would be nice. IE: Are you a pro, prosumer or just a hobbyist and do you take you work with you a lot when and if you travel?
The display is key for what you are doing. I am a photographer and want the best display so I recently picked up a NEC display (NEC 2690) that is non-glossy, wide gamut, extreme shadow detail... If you want something like that then pass on the iMac. I just sold my iMac and 13" MBP to get a 15" MBP and the NEC. With this combo I can do anything - travel and do the best image work and printing. I don't need the power of the MP nor can I afford or justify it