Carbon Fiber is somewhat lighter, it has its unique "look", it generally has been criticized for having a slippery feel. It is generally more expensive than Micarta scales.
Micarta has been in use longer, if bead-blasted it is more grip than CF and sometimes has a distinct "linen" look.
Both are very stable materials and will perform well. If you have a corrosive environment (i.e. certain chemicals) you should look at website of vendors that sell the material for specific chemical tests.
My opinon is CF for smaller knives (classier) and Micarta for larger knives. Some makers, notably the Swiss maker Klotzli has done some great things with CF. Look at their Walker and Wimpff knives (sold through Boker), they have a red/black CF scale material and also a chartreuse/black CF scale material. Awesome.
