You can rub steel clean to get the rust off.
Carbon fiber can't be fixed.
A bike friend was in an accident recently and his carbon fork shredded into a
million shards. Meanwhile, the steel in the rest of his bike bent.
the steel could be bent back, the carbon, well, he's lucky he didn't get stabbed with it.
I also have heard complaints about carbon parts just breaking under stress. The thing is, carbon is expensive. If your steel part breaks (which isn't going to happen) it's going to cost a lot less than the carbon part.
I am feeling very guilty for letting them talk me into buying a carbon fork
instead of a steel one.