Here are two 100% crops... If you down-res the the 5DIII photo to match the 16.7mp of the 1DsII, it will look sharper. Or enlarge the 1DsII image to match the 22mp of the 5DIII and it will look less sharp. The 5DIII is presenting a bigger image of the same scene, so it looks less sharp. If you had a 36mp photo of the same scene and viewed it at 100%, it would look even less sharp. Also the unfortunate moire effect in the 5DIII photo is usually a result of greater resolution, not less.
Scroll them to the right to see those windows with white blinds:
real usable detail in 16.7mp 1Ds MkII files,
mushy moire in 22mp 5D MkIII files.
This really makes no sense.
If I have tiny real details and texture in the white blinds on the windows in 1DsII sample
and I only have moire in the 5DIII sample,
why do you think that just by downsampling the 5DIII file to 1DsII file size, the detail that is not present in the 5DIII file will magically appear,
...or that just by upsampling the 1DsII file to 5DIII file size, the visible detail in the 1DsII file will magically disappear?
The resolved details are present in the files or they are not.
You can just try what you have suggested - use the crops that i have posted above,
resize them both way to match each other
and see if the lost details will appear in 5DIII file when you downsize it,
or if the fine resolved details will dissapear from 1Ds file by upsizing it.
Except for the moire effect, I don't see detail in the 1DsII image that isn't in the 5DIII image. Moire is a problem, but you are mistaking moire for lower resolution. Moire results from interference between detail in the scene and the grid of pixels in the sensor. It is more likely to occur with higher
resolution. Higher resolution cameras such as the D800E are a magnet for moire, much more so than say the original 12mp 5D.
It is fact that when you view both images at 100%, the 5DIII is presenting a bigger image of the same scene. If you could compare the original 12mp 5D to the 22mp 5DIII, both at 100%, the original 5D would undoubtedly look crisper. This is a fact of life of higher resolution cameras: as the resolution gets higher, the images look less crisp if you keep looking at them at 100%
Here is your 1DsII image up-ressed to match the 5DIII image (still at 100%) below it: