I use both organic cotton (cotton balls) and cellucotton (rayon), as the spirit moves me. They're both good; rayon actually wicks up juice a little better, and used in an RDA, can really deliver the nic hit, though I don't notice much diff in the nic hit in a kayfun, between cotton and rayon. With cotton, if it looks like it might not be quite enough wick, it probably is. With rayon, if it looks like it might be too much wick, it's probably not -- with liquid absorption, cottons swells, rayon compresses.
Try both, see what you think. I don't really notice much difference in taste between them, other than in how dry hits from each of them taste -- the "metal" dry hit, from not enough wick, taste much the same, but the kind of dry hit you get from too much wick, or the wick not wicking up the juice fast enough, rayon's is far worse, it's very chemical. The cotton kind just taste like you set your jeans on fire and then sucked on them.
Andria