I opted for the iMac/MacBook combination and am really pleased with that decision.
The 20" screen on the iMac and the full size wireless keyboard is much more satisfying to me personally as a home computer, and in terms of performance, it is actually a bit faster than the MBP I think as the graphics speed has not been clipped out of concerns about heat and battery life.
The 13.3" MacBook, at least to my view, is a much better size for a portable.
I know that people love their MBPs, and I am sure they are lovely, but to me, the screen size is a bad compromise for home whereas the 15.3" is just a little big for the road. Why compromise? As you point out, for the same money you can have the best of each world in its appropriate setting. iMac + MacBook = Joy.