Hi guys,
I thought I share my experience with you, that I had with the Canon 5Ds and the Nikon D850, having used both for over a year now.
Back in 2015, after my old 5D Mark II got destroyed in an accident, I had a big dilemma between the 5D Mark III and the new, super-high resolution 5Ds.
At the end of the day, I decided to go with the 5Ds.
But as life goes, things change, and some of my old friends (who we shared the Photography hobby with) moved to different places, or just changed a hobby, and so in those 2 years I ended up with most of my friends using Nikon.
Back then in 2006 when I purchased my first (D)SLR I had to decide between the EOS 400D and the Nikon D80, I was suggested to buy the brand, that my friends have, as there is really no big difference at that level. So I went with Canon.
But a year ago, I realized that most of my friends now have Nikon, I decided to move on. And after doing a lot of research, I went for the then brand new D850.
I was not sure how the transition will work, what to expect, I never changed systems before, used Canon for 11 years.
The Menu.
First major difference, I noticed, is the Menu.
I must say, in terms of the Menu, Canon is light-years ahead of Nikon. I use the D850 now for a year and still hate its menu.
Its just bad, compared to the Canon. The Menu of the Canon cameras is so much more intuitive, so much easier to use, it just works better.
Now I do not wish to say, that the menu of the Nikon is not usable, or similar, but the Canon menu is much better set up, its much easier to navigate in it.
Its a more effective menu, much better designed.
Menu design for me goes to Canon, by light years.
But thankfully, you do not really need to go to the menu on any of the cameras.
Ergonomics
The first problem you will face when moving from Canon to Nikon, is that everything turns the opposite way.
This is not a big issue though, and you will get used to it in a few days.
When I first held the Nikon in my hand it felt for me a bit more robust then the Canon.
As for the grip, they are about even to me, the Nikon feels a bit better in my hand.
*Update: I recently had a 5D Mark IV in my hand without the Battery Grip, and I noticed, that it is slightly smaller then the D850, but that 5mm of a difference means in my case (with average sized hands), that my little finger is not able to grip the 5D Mk IV properly, it ends up below the battery compartment, while I can grip the D850 with all my fingers. Feels much more safer for me.
One thing I noticed early, is that it is much more fun to switch on the Nikon. The On-Off switch just feels better to use. (It is below the Shutter button).
Yet I found myself sometimes accidentally switching off the camera, that never happened with the Canon. I think this happened like 3 times to me in a year, so its not a huge issue.
The main buttons (Shutter, Aperture, and ISO) are on both cameras easy to reach, easy to use. One small + for the Nikon is that the ISO button is much bigger, then on the Canon, and much easier to reach. (For Nikon users who do not have a D850: The D850 has the ISO button just behind the shutter release button. This in my opinion is ergonomically a HUGE bonus compared to the D750 for example).
As for all the other buttons, like AF, Bracketing, and such, I do not understand their placement on the Nikon, its just not intuitive, you have to look all around the camera. This is much better done on the Canon in my experience.
I do not talk about the menu, and playback buttons, as they are effectively the same.
One of the biggest things I miss on the Nikon is the dial on the back of the Canon cameras. I think it is something that is again much better done on the Canon, the "multicontroller" on the Nikon works well, but the Canon has a much better design. The fact that you can turn the dial, makes it so much "better" to use, then pressing and holding the controller on the Nikon.
*Update: After a year of usage, I accidentally discovered, that you can also use the back and front dials (the same used to adjust aperture and shutter speed) to scroll between the pictures, just like you can use the back dial of the Canons for the same. This is essentially a life changer for me. It is just so good. - The reason I did not know this is that I discovered (basically was intuitive) that the multi-controller on the back scrolls between the pictures, and well, I never tried scrolling with anything else, did not think there are more options. But there is, and thanks Nikon for it! 
What I like about the Nikon is the dedicated dial/button set on the top left of the camera. You can easily adjust a lot of things like (White balance, Picture Quality, Metering, Burst spead, timer, etc.) without having to go into the menu. Canon also has these as buttons on the Right side, yet here I think Nikons approach is a bit better.
Also Nikon has a dioptre correction dial, that you cannot move by accident, and this si quite irritating on the Canon, as I regularly adjusted that on my Canon by accident.
The dedicated Joystick to select and move the AF points on the back of the D850 is a nice feature that I do not find on the D750 for example.
This is a huge bonus to the Camera itself.
For me, the ISO button on the top and the Joystick at the back are two of the biggest additions to the D850 compared to the D750 and D810.
Overall both cameras feel great in your hand, and after you learn where to find the things you need, I would say, this would be a tie.
IF, and only IF the Nikon had a similar dial at the back as the Canon has. <-- Never mind that, it has. (read above.)
So after discovering that there is actually an option to use the back dial just as on the Canon (although its located elsewhere) this i would rate as a draw.
Yet a camera is built to make pictures, good ergonomics and a well designed menu are nice, but are not that important in the long run.
Sharpness
I found that the pictures taken with the Nikon are just sharper.
This is a slight, but noticeable difference. When you look at the sharp areas of a picture (ideally where you focused), the Nikon is just sharper.
There is more detail, in the sharp areas, and the edges are even sharper.
This is true at f/1.8 and at f/8 also, no matter if I use the fix 85mm lens or a 70-200mm.
I think this due to the fact that the Nikon is missing the Low Pass filter, that is used on the Canon 5Ds (and also is missing on the 5Ds R).
As Sharpness is (for me) one of the most important points of a picture, this is a HUGE plus for Nikon.
Image Quality
I would say, Image quality, at ISO 100 is equal, I cannot see any difference.
As for higher ISO settings, ISO 800 is ugly on both cameras. (When zoomed in to 100%, and compared to ISO 100)
One small advantage of the Nikon is that is supports ISO 64 natively, and this enables you to create even more clean pictures then at ISO 100.
Resolution is mainly about the same, so I will not comment on that.
Dynamic Range... Well exposing a picture properly helps a lot here, no matter the camera.
Overall I would say, this is a Draw.
Auto Focus
This is one of the most important features of a DSLR for me.
Manual focus is nice, but sometimes you just do not have time to focus manually.
There is a very important difference here between the Canon 5Ds and the Nikon D850.
The Nikon has additional cross-type focus points at the very edges of the AF Point Area, while the Canon has these Cross-type points a bit more inside of the AF point area.
See below: 5Ds on the Left, cross-type AF points marked in Bold Black. | D850 to the right, cross-type marked with red squares.
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For portraits, I very often use those most outer AF points, to be able to focus on the eyes of the model, while composing the picture.
I found myself quite a few times struggling with the 5Ds, as the Non-cross-type AF points either struggled finding proper focus, or simply missed. Now since Nikon put the Cross-type AF points to the outer parts of the AF point area, this is where Nikon wins for me big time.
An average of 6-7 out of 10 pictures were tack-sharp on the 5Ds. (Focused on the eyes)
This on the Nikon is around 8-9 out of 10.
And at least in my experience, this makes a HUGE difference!
So the winner is the Nikon in this area.
Video, HDR, Extremely high ISO
I do not use those features, so I cannot comment on them.
Sound
Hereby I refer to the sound the mirror makes.
This goes to the Canon by far. Its much more silent, and imo has a much nicer sound overall.
Whenever I hear a Canon 5Ds or 5D Mk IV, I notice instantly how much more silent the mirror is in the Canon.
Overall:
For me, because of the more reliable AF system (well placed Cross-type AF points), and the extra sharpness because of the missing low-pass filter,
the Nikon D850 is the clear winner of the two.
Even despite the fact, I still hate its menu.
Now, please keep in mind, this is only my opinion, that has been formed based on the needs I have for a camera.
Others might be totally disagreeing, or agreeing with what I wrote.
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Addition:
Canon 70-200 f/2.8L IS USM II vs. Nikon 70-200 f/2.8G ED VR II
I owned the Canon before, and own the Nikon now.
So I thought I share some of my thoughts about them too.
The Canon feels bigger, is white, and has a red ring, while the Nikon feels is a bit smaller, is black, and has a golden ring.
Both are weather sealed, both are amazingly sharp, both have rounded aperture blades thus both produce stunning bokeh, and both focus fast and accurate. I really cannot pick a winner here both are amazing lenses, and I rate them as the BEST two lenses I ever used.
The minor differences:
- The tripod collar cannot be removed from the Nikon, but you can easily detach the plate from the lens by loosening it, and pressing one little lever, and goes on the same way. On the Canon to remove it, you have to dismount the lens from the camera, loosen the tripod collar, and remove the whole thing towards the back of the lens.
I much more prefer Nikon's approach here.
- Lens hood. The Canon has a fabric inside the hood that gathers dust, the Nikon does not have that. I do not really feel this as a big difference.
On the other side the Nikon has a really good way of fixing the hood on the Lens. While you can twist the hood on the Canon accidentally, this is not really possible on the Nikon (well it is but you have to apply a LOT of force). I much more prefer the Nikon's hood.
Very minor differences:
- IS/VR. I do think that Canon has a slight edge in terms of Image Stabilisation over the Nikon (VR), but I have not been able to really test it out, its just how I feel. Another thing that I like more about the Canon IS, is that its less noisy. (The Canon is still noisy, but the Nikon is a bit noisier.)
- Zoom ring. I feel that the Nikon is a bit more tight, and if you listen to it closely, you can hear the ring rotating, something I did not experience on the Canon.
- The Nikon is black. (Sorry, had to include that, to show how minor the above 2 differences really are.)
