I think the design, handling, etc. were pretty much the same between those 2 model years. I think Kawasaki just changed the front spring adjustment design just to make it a little simpler and easier. The shape of the ramp that the spring rests in for the 79 skid did provide a more progressive spring rate as opposed to a more linear rate of spring compression on the 78 skid.
In fact, I seem to remember some mention of this in the brochure for the new 79 models...this new design at the front of the skid added a progressive spring rate to the front end, like the progressive rate offered by the clever coil over shock and bell crank design for the back end of the skid.
I doubt that this change to the front spring made any huge difference. Far more important to the ride was the rear spring set-up...which is the same 78---79.