I have both and shoot a little bit of everything but mainly sports. Obviously I use the 1DX for sports but even when I am shooting something other than sports, I first reach for the 1DX and mainly just use the 5D3 as a back-up or second camera. Last week, I shot a senior portrait and used the 1DX. I do use the 5D3 for video though.
I think we're all agreed that if we owned both bodies at the same time, the 1DX is obviously the better performer and would be the preferred choice on most occasions, while the 5D3 would be just about as capable in less-demanding environments.
the 1DX is obviously the better performer and would be the preferred choice on most occasions,
Hi dash...I don't share that view.
Again, I have and use both. Regarding IQ....
In good light for slow or non moving subjects, the IQ is about the same.
Don't get fooled into thinking anything different.
If the light is poor, then the 1dx is just slightly better if you need to shoot at roughly ISO 2000 and above. If you can get by with ISO 1600 or lower, there's not much difference in IQ. (I've looked at many thousands of images from both in low light because that's mostly what I shoot)
So with those two statements made, the need for AF speed and FPS in fast action are the more significant advantages of the 1dx. Along with weather sealing.
I was at a charity event recently where a pro photog was shooting the 1dx while speakers were talking....EVERYBODY would turn and look at this guy when he would fire that 1dx. The 5d3 silent shutter is a big advantage in many situations.