Figure an aloft weight reduction of 40-50% with standard carbon, and a 65% weight reduction with HM carbon. This may or may not be important to you, but it can make a substantial difference in the stiffness of the
boat, in large part because the weight is carried so far aloft.
Many small (moth, windsurfers, A-Cat's) masts are just clear coated, but for a
mast that isn't protected they should really be painted to protect the
epoxy. That being said, painting a carbon
mast is much easier than trying to
paint aluminum, much more like painting
fiberglass. Over time it may become necessary to repaint, but it isn't a regular
maintenance item.
Overall I would always prefer CF to
aluminum however the
price (while getting more affordable) can still be substantially more.