The 1DmkIIN has 1.3 crop factor, and a machine gun burst of speed !
The 1DS has the resolution and when using AI Servo seems not much slower than the 20D because the 20D doesn't seem to get up to full speed while autofocussing.

My 1DSmkII has a much noisier shutter than the 1DmkIIN, after the clunk and mirror slap there is a whirr as the whutter is re-cocked that's missing from the mkIIN.
If you are stalking your wildlife it might make a difference, or require some muffling.
Does the 5D sound the same ?.