The teardown on the 7d2 and 5d4
show the build and sealing to actually quite good.
The 5d4, resized down to the 1dx, seems to show a bit less noise at high ISOs, at least easier to clean up. Less is less, not equal, but yes, with normal viewing conditions, they are close. I just think for a family man shooting a myriad of things, the 5D4 is just a better all around camera, there are just so many more features for family shots, kids sports, etc.
Regarding low light AF, the 5D4 has 1 EV better low light AI servo, -3 EV vs -2 EV. The 1DX shares the 5D3 performance in this regard. I would love to see what others have to say, but I have been able to track stage action in low light with no issues on the 5D4, something the 1D4 was terrible at. I am glad the newer 1D series perform better than the old though. Of course these are published specs, so indeed, perhaps the 1DX can track servo better than the 5D4 for stage presentations or other low light events, despite the 1EV advantage of the 5D4.
Another 5D IV improvement is the 1 stop better low light AF capability, now rated to EV -3 in One Shot mode. Being able to focus in 1/2 as much light as with the 5D III should not go un-noticed by those shooting in light-starved venues, especially wedding and event photographers. The identically-rated 1D X II's center AF point focused on a label (with the necessary contrast) in light levels calling for a 20-second exposure at f/2.8 and ISO 200 using the Canon EF 24-70mm f/2.8L II USM Lens and the 5D Mark IV matched this performance.
Pixel Perfect wrote:
In very low light 5D4 is better I found. I was getting snappy AF in near darkness with centre point using 500 f/4 + 2x @ 12800, well after sunset. Whether that hold sup for tracking, but I have no doubt 5D4 will AF in lower light than 1DX. I found the 5D4 was excellent in AI servo as is the 1DX of course. Haven't had a chance to do a rigorous side-by-side, but after several hundred shots at X-mas with waders and BIF's I'm very happy with the 5D4. It at least matches the 1DX in most cases I had and it is clearly better than 5D3, which I found wasn't as good as 5DsR and a fair way behind 1DX in many situtations, although still good.
Adobe clearly changed profiles for the 5DsR and 5D4 and need different settings than the older 1DX or 5D3. I've found LR 6 overly constrasty and use totally different settings now than I ever used to need. So IMO when I open 5D4 in LR it I need to bring constrast down a lot more than with 1DX, I'm often using -30 as opposed to -10. I haven't really noticed less saturation though.
I'm using DxO more and more for PP now as the I can get better detail out of it (lens profiles are better) and it rivals DPP with DLO for detail extraction and it's PRIME NR blows DPP and LR out of the water. If only they would implement local adjustments. I have just isntalled trial of C1 10 so will see how it compares.
Also important, something I left out, is the all 61 pt f8 focusing. I am not sure what the 1DX offers in this regard, but the 5D4 offers f8 AF.
Especially important to wildlife photographers is that the 5D IV supports AF with f/8 max aperture lens combinations (a lens plus an extender) at all 61 individually selectable points with all 27 central AF points providing cross-type AF support. Version III extenders are required for this functionality and most current lenses are fully supported, though there are some limitations as noted in the manual.
One more sample, showing the advantage of the 5D4. Due to the resolution increase, you can post process the 5D4 to the point that once you equalize the two, the 5D4 shows more detail and less noise. The resolution and noise signature of the 5D4 creates better post results. The 2 images here show what the 5D4 has over the 1DX for resolution, then post processed and equalized. At 200%, the differences are noticeable.
In the end, one could probably just flip a coin and get a good camera, no matter what. Not everyone pushes their equipment to the extremes. 
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Used 1dx
