Cotton for flavor and wicking ability, cheaper to maintain and easier to rewick, easily molded when wet.
Silica if you are allergic to cotton (heavy feeling in lungs after long periods of
vaping, etc). It works well, but the biggest strong point in a Kayfun, if you are using the whole metal
tank assembly and can't see
juice level, silica won't burn if it gets dry, and is easier to clean instead of rewicking all the time when it gets nasty.