I had a 1D MkIV and a 5D MkIII. I used the 5D for most things and the 1D For my main commercial activity, shooting bodybuilding shows. I had used this approach thru previous generations (1D MkII, 1D MkIII, 5D MkII). It worked well for me.
I need screaming speed for bodybuilding shows. I liked the resolution for most other photography such as burlesque shows, live band performances, and the odd model shoot.
I got a 1Dx and sold the 1DMkIV. I found that I no longer carry the 5D for anything. I only shoot the 1Dx.
So I am looking to sell my 5D MkIII.
Once I realized the low ISO capabilities of the 1Dx the 5D MkIII became the rarely used backup camera. The 5D is a great camera. The MkII was too. But as the 5D MkIII is much better than the MkII the 1Dx is honestly that much better than the 5D MkIII.