1DSII is full-frame, meaning I can at least theoretically get a little more performance via slower shutter speeds without shake (using 1/f general rule of thumb). But the ISO limit is 3200 (H). Also I already have a 1DII and am familiar with its (somewhat peculiar) button-pressing scheme...plus the 1DII and 1DSII use the same batteries.
On the other hand: 1DIII is APS-H (1.3x crop), ISO max is 6400. Plus it has newer DIGIC processors. But I know there was some concern about auto-focusing of the 1DIII (maybe very specific conditions?).
My use will be widely varied, but a big portion of what I do is dance photography (on-stage, with poor lighting) and it's one place where the camera/gear really does matter.
It's not too hard to find either of these bodies used, and the price is comparable. Which one is likely to serve me better?