I have a 1DMKIV that I use with the 300 f2.8 IS II for aviation photography, and I'm very happy with this combo.
But I want a second body to use with another lens. I can buy a used 1D4 or a new 5D3 for slightly more money. I've read user reports that say the 5D3 can AF as well - or a bit better - than the 1D4. The 5d3 has a smaller memory buffer than my 1D4, but since I don't do much continuous shooting, it should not matter.
The big difference is the sensor size - 1.3 crop vs FF. If both cameras used the same lens, I would have to crop the 5D3 image down to 13 mp to have the same fov as provided by the 1D4. So the question is how well will the 5D3 image quality hold up when cropped down to 13 mp, and printed at 8x10" size?
Some drawbacks of buying a used 1D4 are...high shutter count, no warranty, battery may be towards the end of its useful life, and the camera may be more prone to need servicing. On the other hand it is a great sports camera.
Any opinions are welcome.