The SL6 have both a side and rear switch, whereas the SL6S range has only the side switch.
Only mike many lights where two switches would have different functions (on/off, and mode switching for instance), with the Spark lights, both switches both turn the light on, and switch modes, but you have buttons in different locations to suit different holds.
The one downside of this arrangement is that there is a thin tube inside the SL6 body to make contact between the front and rear switches, which can make some 18650 cells a tight fit inside the light.