If you look at the waxless patterns of the two skis you'll notice that they are quite different in construction. The Fischers scales are cut into the ski base, the Karhus are raised from the base. As a result, the Fischers will glide better but climb worse than the Karhus.
It's been my experience that in moderately difficult snow conditions (cold, loose snow, or cold hard snow) the Fischers have limited grip. The Karhus will definately grip better. Most folks I know who have the Fischers carry skins because climbing is really not fun in difficult conditions. I don't know too many folks with the new Karhus, but not as many carry skins.
Is this worth new skis? Possibly, depends on the kind of skiing that you do. If you are interested in turns, the Rebounds are clearly the better ski. How much of your skiing is on rolling terrain?
-dave-