The general rule is that larger pixels can capture greater dynamic-range. This of course would be a disadvantage for the 5DS R expect that technology has improved. Despite the smaller pixels which can hold fewer photons, there is less noise in the circuitry within the 5DS[R] sensor. The result is a greater dynamic range by slightly more than half a stop.
Should you want even more dynamic-range from a single shot, you can by getting the Nikon D610 which offers a full 2 stops more than either 5DS or 5DS R at base ISO. There are other DSLRs to attain similar dynamic-range, namely the Nikon D810, D800 and D750. For high ISO, I found that the Nikon D4S had the widest dynamic-range.
You mention Sony which unfortunately I have not reviewed yet, so I cannot tell if the dynamic-range is comparable but we do know that Sony and Nikon share sensors which indicates they should be close in terms of RAW performance. You may check with DxOMark which measures dynamic-range and other metrics independently from manufacturers, although I find them inconsistent with my results in the last two years.