As I await for my MBP, before I ordered my MBP I also thought about either getting iMac or else the same scenario here with the MBP and a external display. From the spec on the Apple page, which I check everyday 😛 the MBP and the iMac has pretty much the same spec. Besides the iMac having a larger hard drive stock they are pretty much identical in spec. They both have a intel c2d 2.16/2.33, both have ATI X1600 video. I am though comparing the 20" iMac and the combination of the 15" so it might be a little skewed in comparison.