At least for me, the determining factor here would be how you want to interface with the camera. The RX100 operates like a traditional compact - power zoom (you can use the control ring on the front to zoom though), no viewfinder, menu/button driven interface, whereas the X20 operates more like a traditional interchangeable lens camera (even though it isn't) with a viewfinder, manual zoom, dedicated exposure compensation dial, dedicated buttons/control wheel, etc. They are two different beasts in that regard and as such its difficult to just compare features.
RX100 bigger sensor+slower lens in a smaller body (jean pocket) vs X20 smaller sensor+faster lens in a bigger body (coat pocket). I think both are very good cameras depending on your needs. You can check the X20 comparison with X10 and RX100 at quesabesde: