Have both of these and my experiences are as follows:
S100 is definitely more pocketable than the RX-100.
S100 UI is easier to use and navigate than RX-100.
S100 is much cheaper than RX-100.
RX-100 is better at low light.
RX-100 IQ is MUCH better.
RX-100 feature set improves the functionally of this P&S.
In summary: RX-100 goes in the bag. S100 sits alone on the shelf.