Upload & Sell: Off
I own both the 1Ds MIII and the 1DX. I have ~15k shutter activations on the 1DX.
80% of my shooting has been/is performance dance in low and/or extremely high contrast situations, awash with shadow and everyone spinning like dervishes. :-)
Between the two bodies, in my use cases, there is absolutely no comparison as the 1DX is vastly superior. The issues are AF accuracy, high ISO and Servo tracking predictability.
I loved my 1Ds MIII from the day I bought it in 2008. But it will soon be for sale. It is a spectacular body, but for my uses, it can no longer provide what I need.
I owned a seminal 5D, but have never used the newer 5 bodies. I am sure they are great, and at a great price/performance ratio. I would not hesitate to recommend one of those for most people.
But I am fully "in like" with the build quality of the 1 series after random (on a shoot, on vacation, just stuff which happens ) accidents over the last 5 years which resulted in zero functional damage. I'm not abusive of my gear, but I do treat it like mechanical gear, not trophy pieces.