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The 1DsIII has a pixel density of 156 pixels-per-mm (ppmm), while the 1DIII has 138 ppmm. This means that an image from the 1DsIII will have slightly higher resolution if it's cropped to the same 1.3x CF image size that is produced by the 1DIII. The 1DsIII image will also be slightly larger in mpx than the 1DIII image, as it has more pixels. This assumes you're using the same lens and same distance to the subject with both cameras, resulting in the same image content projected onto the sensor. There's no particular reason to crop to the same mpx, it's usually more useful to think of comparing the same size image or the same size of output (i.e. screen size and print size).
The 1DIII provides higher frame rates and a larger image in the finder (the 1DIII finder is actually smaller, but has the same AF point arrangement as the 1DsIII), which means the AF should generally be more effective for relatively small subjects (e.g. BIF).
If you don't need the high fps, then the 1DsIII is probably your best bet. It's also an incredible all-around camera. The 1DIV is even better for wildlife (i.e. for cropping), as it has the highest 1D-series pixel density of 175 ppmm. The 1DX is in between the 1DIII and 1DsIII at 144 ppmm.