I did some testing with the 1Ds3 and the 5D. 1Ds3 in Highlight Prio, both cameras RAW. Parts of each of the images were overexposed by the same amount, and I brought them back by -2 stops in DPP to see about highlight retention on these two cameras. I'll also try C1, ACR, etc. and post results at a later date.)

Used the 50 1.4, tripod and strobe, ISO 200. Exposure was 4.5 at 1/125 sec.

1Ds3 on the left, 5D on the right.

While highlight retention is a lot better on the 1Ds3, resolution did not look a whole lot different between these two cameras when shooting in daylight on a tripod. However, shooting with strobe showed a more significant resolution advantage for the 1Ds3)

More samples will be posted soon.