These exposures were during an actual shoot, using the same exposure, ISO 400. (highlight prio enabled on the 1Ds3)Processing was the same,used DPP. (1Ds3 left, 5D on the right)

By looking at the stools on the left, you should be able to see the difference in highlight retention.

As to resolution differences, in my experience the resolution differences are really only visible when using strobe. Shooting these cameras side-by-side using daylight with the same lens, for the most part I really can not see the resolution boost in the 1Ds3. Strobe shows the differences a lot more clearly though.