Few points to consider:
* Basically, the two cameras are identical, expect for:
- MPs, obviously
- Cards: IDsIII can use UDMA while 1DIII can't
* Judging from your work (if memory serves) it doesn't seem that you need the high frame rate (which you pointed out already)
* Need Max's input on this one: Noise at high ISO. You probably live in the high ISO range to get the shutter speed you want or for being in the shade. This could be critical to the decision, imo.
* To counter the above point: The 1DsIII files can take a lot of beating and still smile. It's amazing in this regard.
* The details coming out of the 1DsIII will make you dizzy
Just some thoughts