Both tulle and organza are sheer, but tulle is softer and more net-like. Organza is a more sheer and woven fabric. It is also more smooth than tulle which has a mesh-like tool finish, and is more shiny.
This is important for even more shifts of diaphanous sleeves and other structures. For these simple reasons organza fits every designer's ideal, while tulle is more fit to skirts, skirts, etc.