AF AF AF of the 5D mark iii.
Try it yourself.
Yes I know the 1Ds is also good,but please try the AF of the 5D mark III.
For me worth the upgrade !!!!
Yes the 5D's AF is reported to be good. But the 1DS drives lenses at a higher voltage (from what I understand), meaning they physically respond more quickly. Unfortunately you don't gain this capability when adding a battery grip to the 5D either.
I have played with a couple of 5D IIIs in my local camera shop, but I've never handled a 1DS (I don't know anyone with that much money to spend on a camera). But considering I shoot with a 1V film camera, which is built the same way, I think I'm comfortable with the general feel, and I vastly prefer the push buttons on the top of the camera rather than a dial.
The biggest factor in the 5D's favour is its low ISO capability. Looking at comparison images of ISO 1600-3200 between the 1DS and the 5D Mark 2, even the Mark 2 outperforms the 1DS in some situations, so I can only imagine what it would look like comparing it with the 5D Mark 3.