Essentially, the size of the dust container and the water tank are different: The S6 has a larger dust container, while the S6 Pure has a larger water tank. All other differences are just hardly noticeable. The S6 Pure is a kind of light variant of the S6 and should therefore usually be a bit cheaper than the S6.
Here is an overview of all the differences:
|Dimensions||35 x 35 x 9.65 cm||35.3 x 35.3 x 9.65 cm|
|Charging station||smaller than the predecessor S5, without cable management, underlay pad less stable||larger, with cable management and more stable attached underlay pad|
|Buttons||Spot cleaning can only be activated via key combination||exclusive spot cleaning button|
|Sensors||4 sensors, rear crash sensors missing||6 sensors|
The manufacturer states that a couple of the materials are different, which probably explains the weight difference.
Visually, the two look very similar: