This winter, I've been riding a Salsa El Mariachi frameset. This includes their Steel fork. I also have a niner Air9 that's currently set up with a White Brothers Rock Solid. I havent been on the niner in months... I had built it up rigid as a pit bike for cross season, and I might have used it once since building it up. Anyway, I'm starting to ramble... I just rode the Air9 yesterday on a seriously rocky trail system(French Creek, PA). the carbon fork felt like it was a lot stiffer than the steel one on my salsa, for sure. It vibrates a lot more as well... The vibrations though, dont seem to affect handling... its more when I hit something, the fork flexes and springs back(forward). I think the steel is generally more compliant than the carbon and I think I actually like the steel better.
Its tough to judge the weight difference, as they are two completely different bikes. The steel salsa single weighs 23-24 lbs where as the scandium niner with gears weighs close to 21lbs I think. The frames obviously have different ride characteristics as well... the steel is a lot smoother and the scandium is a lot snappier.