In short the 5DmkIII allowed me the space I needed to get stuff in the frame, where the 1.3x crop of the 1DmkIV was too much the 5DIII took over. I would love to say it is fast and amazing but the 6fps are just a touch too slow, the buffer is a touch too small and the batteries do not supply enough power to the AF module so it is not as snappy as the 1D series.
That said the AF is customisable, accurate and comprehensive. Really quite impressive, I can only dream of what the 1Dx is capable of. 6fps is probably enough for most situations but these buzzards of mine are quick and I miss having those inbetween frames you get at 10fps. But OK, the 5D AF is accurate enough to nail you important shots, the unfocussed ones are down to user error. Me basically.
I was shooting at ISO400, so I will not coment on IQ just yet but the images are full of full frame goodness and at ISO400 there is no noise to speak of.
It is a very balanced camera, at long last a 5D which is useful for more than just snaps.