Thats interesting, I thought the 5d was sharper than my 1ds mark 2. So do you think the 1ds mark 3 is sharper than the 5d? Looking at the 1ds mark 3 files, they seem a little"thicker" for want of a better word maybe than the 1ds mark 2 or 5d files. Do you find this to be the case?
The AA filter on the 5D is less aggressive I think then those used in the 1D Mark I/II/IIn, and all the 1DS series bodies, and for this reason, the 5D raws seem a tad sharper.
However none of these bodies produce too soft images that cannot be mitigated with proper USM application.
I would be a bit more concerned with the lenses used then the body in regard to sharpness.
The 1D/1DS pro bodies are less aggressive with in-camera sharpening I think, allow the photographer more room for his decision making, which I think is the best way to go generally. The less aggressive AA filter on the 5D means a bit more moire, and other digital artifacts, but this is not really a problem. I have found the 5D showing some banding in skys and walls however. Often cordoray and tight knits/patterns can show some moire, but applying AA in post can often mitigate this issue, the few times it happens.
My experience with the 1DS Mark III is not much, as I have rented one and made about 1,000 or so exposures, and what I found was that it made superb awesome images showing noticably more detail in for example wedding dresses and fibers in tuxes then the 5D provided. The highlight management with the Mark III proved wonderful, and yest "thicker" too. ;-)