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.
5D3, 6D, 1D4, 16-35 2.8L ll, 50 1.2L, 50 1.4, 24-70 2.8L II, 135 F2.0L, 200 F2.8L ll, 70-200 2.8L II, 70-300L, 100-400L, 24 TSE L II, 85 1.2L II, 100 2.8L, ZEISS 100 2.0, ZE 50 2.0, ZE 35 2.0, ROKONON 35 1.4, SIGMA 35mm f1.4, SIGMA 85 f1.4, S110, S100, sony RX-100 and one big-a$$ bag