I have the 5D3 and an older 1D3, the 5D3 replaced the 5D classic I sold and the 1D3 for the most part (1D3 is now my backup camera). The 1D series body is more bullet proof for sure, but the 5D3 is no slouch either. The biggest issue I have with the 5D3 is in very low light, the AF seems to struggle to attain lock. Once it does, it's golden. I would suspect the 1Dx will do better in attaining AF lock quicker in lower light, since it has more processing power, and maybe because the higher voltage battery will activate the lens motor quicker, particularly for bigger glass lenses (i.e. 50/85 f/1.2 and the long lenses).
As for image quality, what I've seen samples of with the 1Dx does show outstanding IQ at higher ISOs, probably better than 5D3 from what I can tell. Of course, it will shoot faster too, so for speed of handling, durability and perhaps better high ISO images, it's obviously the better camera. Whether the difference is worth the extra $3k+ is up to you.
As for low ISO image quality, I suspect Canon is going to release something along the lines of a D800 type camera soon (next year or 2 anyway), which will fit the need for that kind of photography. Generally, the need for super high IQ, high MP, low ISO images doesn't require a 1D series tank for the body, with the corresponding price tag. I think the 1Dx is perfectly spec'd for the market it is targeting. They realized that when the 5D2 killed the 1Ds3 sales, so why build a camera that most people aren't interested in?
