X20 vs X100s @23mm
"Given the rather large initial price difference between the X100S and the X20, other than the APS-C sensor, does the X100S have any significant low-light/speed advantage over the X20?"
Other than the sensor, no it doesn't. They both have fast lenses. If you want to shoot wide open the X100S might have a little advantage. The X10 only goes down to F2.2 at 23mm (=35FF) versus the X100S of 2.0. Since the X20 has the same lens as X10, the F2.2 is probably also true for X20.
If you are asking, if the X100S is a much better street camera than the X20 I would say, "Yes, the X100S is IMHO the best street camera on the market today, now that it has fast AF even in low light and a 16MP X-Trans sensor with no low pass filter."
Here is a "The Camera Store" field test comparison of X20 and X100S that may be helpful: