I've heard two reasons why carbon skis are different to glass skis - the first is weight, but other is stiffness. The way I've heard it explained, a stiffer boat responds more directly to the paddle - flexing absorbs energy so a stiffer boat sees energy translated into speed more efficiently...
From my own experience, I much prefer my carbon ski - it "feels" perceptibly lighter and more responsive and seems to accelerate better. I broke my carbon ski and, while having it repaired, bought a glass one. I figured I'd use the glass one for social & training paddles and bring out the carbon one for races. The truth is I love the carbon boat so much I've hardly used the glass one.
Rob
Currently Epic V10 Elite, Epic V10 Double.
Previously: Swordfish S, Evo II, Carbonology Zest, Fenn Swordfish, Epic V10, Fenn Elite, Red7 Surf70 Pro, Epic V10 Sport, Genius Blu, Kayak Centre Zeplin, Fenn Mako6, Custom Kayaks ICON, Brian's Kayaks Molokai, Brian's Kayaks Wedge and several others...