Organza is great for more structured, luxurious, glossy designs. It is great for fancy outfits because it holds its shape. Tulle is better for layered designs and softer, voluminous looks. For outfits that are more comfy and voluminous, tulle works better. Organza is more suited for snazzier designs; it is also great for bridal gowns. For designs that require more finesse, both fabrics are great.
All in all, it depends on the use of fabric; the comparison never ends, but both fabrics can't outshine each other in their respective fields.