stiff boards are faster as they have the strength (longitudinally) to cut through the snow, rather than bend and flex over it. ie. you need to travel a shorter distance to get from A to B (so to speak)
also on turns, (due to torsional stiffness) they can hold their edge higher at a more obtuse angle and so are less likely to wash out, creating a sideslip which causes a loss of speed and direction.
personally i love stiff boards, but that is coz i am an aggressive rider (aggressive not fast) and for jumps stiff planks often give lots of lovely pop on the ollie.
as for rails tho.... well i leave them for skateboarders so i couldn't give a monkey's uncle!
