Personally, I love LOVE the 1D form factor. If you are going to carry around a big non-pocketable not a phone camera you should go big or don't bother. Crazy great battery life, let the rain fall construction and a grip that lets your pinky get in on the action when you are handholding a 400 2.8 lens that the geezers say you shouldn't.
That said, I'd take a 5DIII over the 1DX in a heartbeat. I'd even take a 1D4. Why?
The 1D4 puts more pixels on the distant object and it does it with great pixel level clean iso performance. The now sadly orphaned APS-H sensor size always had little man syndrome. It was a bit short but it always punched way above its pay grade to compensate. It out resolves even the 5D3. Oh, and it autofocuses (If you are in to that kind of thing) at F8 so if you throw an EF 2x III extender on your EF 600mm f/4 L IS II lens you can mash the button and get the shot.
The 5D3 has the same great autofocus system as the 1X, I'd expect same as-good iso performance and more pixels on the target in a body that can now take the rain.
Frankly I don't know who 1DX users will be, I guess there is a need 12 shots vs. 10 a second (or 14 without focus) demographic perhaps.
