The 5DS / 5DS R are of the same era as your 5D3 so it is an aging platform but still a good one IF you need a high resolution sensor. I bought a 5DS R courtesy of an overseas traveling friend last year when the price first dropped to around $1,500 (these are gray market versions so be aware of that and the lack of Canon warranty when buying).
I primarily shoot with 1DX series bodies but at the price I thought it would be interesting to experiment with the high res sensor and to also have a lighter weight body for casual use compared to the 1DX weight.
After about a year of ownership I am happy with my 5DS R. From experience I found that that with a 50+ MP sensor, motion blur can occur at higher than expected shutter speed so be prepared to shoot a little faster than your typical hand held speed. And to get the benefit of this sensor, you need to use high quality glass.
I have found that I used the camera more than expected and although its high ISO performance is nothing like my 1DX III (or even original 1DX) those are excellent low light bodies and the 5DS R produces very nice images even when the ISO is pushed a bit. This is particularly true when you aren't using the full resolution capability and downsize the image when converting the original RAW to a more normal size jpg.
The RAW images from this camera are huge (some in the 70 MB range) so make sure that your computer is up to processing them. Although it doesn't create the processing load of the 1DX III CR3 files, the 5DS files are noticeably more processor intensive than 1DX II and files from other Canon bodies of that era.
And my 5DS R isn't as light as it started because I missed the vertical grip/controls of the 1 series too much so now it has a Canon battery grip on it so it is about as big as my 1DX series although still lighter.
The first photo was shot with the 5DS R at ISO 6400 (and pushed a half stop further in post) and a small amount of NR was applied during post, it could benefit from more agressive NR but the image is usable for less critical requirements. The second was shot at ISO 200 which better shows what the camera can do with the right conditions.
Rodger
