I moved from a d800 and 200-400 towards canon for wildlife photography,
I also had a lot of doubts between the 5dIII and 1d mk iv
It started with coming down from 36 mp to something lower and lose the cropping ability i had,
I had asked some questions in the forums online about auto focus performance , mixed opinions on that!
Some say the 5dIII auto focus is MUCH much better, others say the 1dmkiv is better (especially in driving the bigger lenses focus motor), it came from people who all had really nice galleries and owned both bodies..., and I interpreted it as : It will probably be close between the 5dIII and 1D Mkiv , while the 1dx will be in a totally different league,
In the end it was fps ruggedness and price that moved me towards the 1d mkiv.
Also it was pointed out that the 1div can drive lenses ( such as 500 f4 ) faster.
Here it was 2400 for the 1dmkiv in really nice condition, a 5dIII is 2800 and then you need a grip with extra battery, so a price difference of like 600+ euros...
So far i have not regretted it at all, the focus feels snappier then on the d800(and that had great auto focus for my standrds) , and the fps is just insane.... from 4fps (6 with grip and dx mode ) in the d800 to 10 is a huge difference.
The only thing that i dislike about cameras is the buffer (when shooting raw).... , 3-4 seconds and its full
I would like to see the nikon d4 buffer on all cameras ( ram is so cheap?)
Also i am not sure yet on which 19 cross type focus points are used on the 1dmkiv ( I currently testing center focus with surrounding or 45 expansion), not sure how much accuracy is lost in that mode (f4 lenses only utilize 19 cross types vs 39 for 2.8 lenses), but the focus on a f4 lens, even with 1.4 converter seems to be really quick for tracking birds ( only tested it on birds so far)
So in general: I would go for the mkiv, and wait for the price of the 5d to come down more, or a higher megapixel camera from canon in the future
The 5d3 might be better at higher ISO, but some people reviewed the images (cropped the 5dIII) to get the same crop factor, and they were quite close there.... with a bit more detail in the images of the 1d MKIV
( link : http://www.dougbrownphotography.com/2012/04/03/canon-5d-mark-iii-vs-1d-mark-iv-high-iso/
I think to someone who shoots sports it should be a 'no brainer' ! , with hindsight I myself don't really understand why i had doubts ( for wildlife) , other then that i came from the nikon camp and didn't know anything about the 1div other then some useless snapshort comparisons:p
10fps+1.3 crop factor , good battery life, a body that feels like you can use it as a hammer ,
Cannot go wrong with it