You don't really have to own the cameras (yet) to be able to compare them to a large extent... Besides, very few people own the 5DS/5DS-R yet. Plus, what users there are have only just received and started to use the new cameras.
In many ways the 5DII and 5DS/5DS-R are more similar than they are different. They are essentially the same body, AF, viewfinder, power supplies, accessories and more. Of course, the sheer 50MP resolution of the 5DS is it's most obvious and notable difference (versus 22MP in 5DIII).
If you shoot low light a lot, stick with the 5DIII. It has has larger pixels and a less crowded sensor that make for lower image noise, wider dynamic range at high ISOs. It's available ISO range goes 3 stops higher, too (ISO 102400 vs ISO 12800). You'll have to be the judge whether or not those ultra high ISOs are actually usable, though. The 5DS/5FS-R are not as well geared for video, either.
The 5DS/5DS-R have a metering system upgrade... inheriting the 252-zone system used in 1DX, as opposed to the 65-zone in 5DIII and others.
5DIII is slightly faster shooting... with a wee bit less shutter lag and 6 fps vs 5 fps shooting speed. As you might expect, the 5DS models can buffer slightly fewer of their much large image files, before they need to pause to clear. The difference is 14 RAW files vs 18 RAW files.
If your work has had you seriously considering a medium format digital from Leaf/Mamiya or Hasselblad or Leica, then Canon has a bargain for you! If you need ultra high resolution for extremely large prints or other reasons, the 5DS/5DS-R may make sense. But you also will probably need to be geared up with bigger memory cards and hard drives as well as more powerful computer hardware to accommodate the more massive files. And Canon themselves has come up with a "recommended lens" list too... Lenses that they deem high enough quality to be used on the 50MP monsters.
Canon appears to have paid some close attention to making the new models quieter than their predecessors, too. That's a good thing in certain shooting situations!
If shooting under fluorescent or similar lighting, the new models have Flicker Sync technology (same as 7DII) to help with that.
I haven't been able to determine if the 5DS models have faster Dual Pixel Live View focus, like the 70D and 7DII do (but the 5DIII doesn't).
Like the crop sensor cameras with high density sensors, handholding with the 50MP cameras is likely to be a bit more challenging.... Might want to keep shutter speeds up slightly or use a tripod or monopod a bit more often.
The 5DS models also have enhanced bulb, time lapse and intervalometer features.
5DS versus 5DS-R is another question... mainly I think anyone shooting architecture or other manmade objects would be more inclined to use the 5DS, while someone shooting natural scenery where there is less likelihood of moiré issues, and wanting that last little bit of fine detail in their images, the "R" might be the camera for them.
Online reviews may or may not be helpful. I often read them anyway... Even reviewers I don't particularly care for and sometimes disagree with can bring out interesting points.
Regarding this comparison, check out:
http://www.the-digital-picture.com/Reviews/Camera-Specifications.aspx?Camera=980&CameraComp=792 That site also has a more in-depth review of the new models.
Also:
http://www.kenrockwell.com/canon/5ds.htmI think those are the two reviews are the most useful... so far.