There has not yet been a study done on wicking materials. However, Dr. Farsalinos, who is well known in this community, has said that if he had to choose which was safest he would pick cotton.
In my opinion, I have issues with silica. And rayon is synthetic, so I hesitate to use it. I use either organic cotton balls or Japanese cotton. And the Nautilus' BVC coils are ceramic.
Until studies are done, it's your choice what you feel comfortable using.