Wow. If you could find an even swap, your 5D Mark III for a 1D Mark IV, I would do it in a heartbeat, assuming the 1D Mark IV is in excellent shape.Couldn't agree more, provided the strengths of the 1D4 vs the strengths of the 5D3 are what you require. I own both and they're a good complimentary pair. On most jobs I'll have the 70-200 on the 1D4 and 16-35II or 24-70II on the 5D3. But if it came to a toss-up, one or the other, it would be the 1D4 every single time. The 5D3 is called on when the project requires maximum IQ (short of renting MF) or complete FF wide-angle delivery.
1-Series bodies are expensive to buy new, but heavy users working in demanding situations know 100% that you get what you pay for. BTW, one sometimes overlooked advantage of the 1D4 is the 1/300 flash sync speed vs 1/200 on the 5D3.