don't think the actual sliding is gonna make a difference...
i think the only thing with soft skis would be that you could more easily butter/ollie/nollie onto the rails, also presses and taps would be a bit easier ....
the only "sliding" that would make a difference would be how wide the center of the ski is.
you would want a softer ski, which would help you for butters, plus its a more forgiving ski so if you land backseat you dont fall out