I do not have a 5Ds/5Dsr but I do have a 5D III and a friend of mine has a 5Dsr.
The body and controls appear to be nearly identical. If we put tape over the badge so that you couldn't know what model it was, you probably would not be able to tell by looking at the control layout.
The 5D III is a better general purpose camera with more overall versatility.
The 5Ds and 5Dsr are specialty high-resolution cameras which are not necessarily suitable for general-purpose use. They tend to be used in situations where the photographer has more control over shooting circumstances. This include studio use, landscape use, architecture use, etc. where your subject isn't a fleeting moment, you can use a tripod, you can use lighting and light modifiers, etc.
Also implied is that you print... and you print BIG!
As was mentioned in a response to another recent post... the difference between the 5Ds and 5Dsr is all about the anti-aliasing filter.
When you take a photo of images have certain patterns in them (grid-lines -- the sorts of things that would appear in rows of bricks on a building, or fabrics in fashion photography, etc.) using a sensor with neat rows and columns of "pixels", then the resulting image would have a moiré pattern. (See: https://en.wikipedia.org/wiki/Moiré_pattern )
To get around this problem, an anti-aliasing filter is present in most digital cameras and this filter is located just in front of the imaging sensor. It substantially reduces the moiré effect, but at the trade-off of also very slightly "softening" the image.
When someone spends a considerable amount of money for a camera to massively increase the resolution of the image, they are probably trying to capture fine details beyond those which a more general purpose camera would capture. In other words... they want "sharper" images.
SO... Canon makes an edition of the 5Ds called the 5Dsr which eliminates that anti-aliasing filter so that you get no softening whatsoever. But this IMPLIES that you understand what that means... if you take photos of images that are prone to generate moiré you are going to be disappointed.
True "nature" doesn't typically have nice neat rows and columns -- and so probably will not be prone to create moiré. For these types of subjects, the 5Dsr can produce images that may be just the tiniest bit sharper.
But if you are shooting "man made" subjects (architecture, products, certain fabrics with patterns or textures) then you may end up with moiré if you use a 5Dsr and for those situations the 5Ds would be the better choice.
And to Bob's point... there's just a little more to it because if you shoot using a high f-stop then you incur diffraction in your images which will limit the ability to resolve fine detail. This is somewhat at odds with the idea of a "high resolution" + "landscape" because usually landscape photographers do want a very broad depth of field (e.g. f/11, f/16, etc.) and the physics of light diffraction mean that the very high resolution sensor is diffraction limited at f/8 and even starting to get diffraction limits at f/5.6. So in other words you'll get the sharpest results when shooting f/4 or lower -- which a landscape photography is unlikely to use. These limitations are based on laws of physics and the math of diffraction assumes "perfect" optics... so no amount of money on a better camera or a better lens will be able to skirt around this limitation. SO... not just "any" natural subject will get a benefit from the use of the 5Dsr.
The ISO performance of the 5Ds and 5Dsr isn't quite as good as the 5D III -- so it's not suitable for low light photography (not unless you've got a tripod and the ability to take longer exposures.)
Also... as was previously mentioned, you are using a sensor with the ability to resolve fine detail and that's not going to help if you use it with a lens that does NOT have the ability to also resolve fine detail. In other words... if you buy a 5Ds OR a 5Dsr you should own a lot of "L" series lenses and resign yourself to only buying the highest quality glass for your camera. Otherwise it would be like buying a very high performance sports car... and putting budget tires on it. Don't do that.
If your images are intended to be viewed on a computer screen, then there's not much point in owning a camera that has resolution far beyond what any computer monitor can display. But if you like printing large images at high DPI then it's nice to have a very high resolution sensor.
One last thing (because believe it or not this actually does happen with these cameras)... people buy the camera... and then COMPLAIN that the images are HUGE and they take too much disk space. High res images take a lot of disk space. If you don't have a lot of disk space or the thought of spending more money to buy external storage or storage arrays is more than you budgeted for... then these very high resolution sensor cameras are not for you.
But... if you understand the storage requirements, love to print your images and love to print big, own high-end glass and don't have a problem with the price tag of high-end lenses, then nothing else on the market can provide the staggering amount of resolution as these cameras.
So while the difference is simple (the presence of the anti-aliasing filter), the implications of that difference are technical and nuanced. The cameras wont just be "better" for anyone -- they would only be better in the right circumstances. This is likely why Bob says "those who need the camera(s) will know that they do."
Tim Campbell
5D III, 5D IV, 60Da
