Ceramic fibre and rockwool are both light, fibrous insulation, but they are made for different jobs.
Ceramic fibre
Ceramic fibre handles far higher temperatures. Its low thermal mass means a furnace heats up and cools down quickly, which saves fuel on batch or cycling operations. It is used as a backup or hot-face lining in furnaces, kilns and heat-treatment equipment, and for door seals and expansion gaps. It is not suited to abrasion or fast gas flow across its surface, and the dust calls for gloves, eye protection and a mask when fitting.
Rockwool
Rockwool suits the lower-temperature side of a plant: boilers, steam and hot-water pipework, ducts and tanks. It is economical and also absorbs sound, so one layer can do two jobs. It is not for furnace hot faces.
A rule of thumb
Choose by the temperature at the insulation’s position, not the process temperature. Insulation deeper in a lining sees much less heat than the hot face. Then set the thickness by the outside surface temperature you want to reach.
General guidance. Always confirm limits against the product data sheet.