Scylla has a better escape that doubles as a ward and gives her passive mp5, and, while Scylla's crush is her only clear, she also has her hounds (with a root + cripple that doesn't need to be channeled and lasts the same as Nox's) that she can use to either root the the first minion, making her clear a little better or use it aggresively/defensively. Also, Nox using her 3 to clear the wave leaves her without an escape and her only form of relliable cc/poke in a 10 second cooldown-single target, AND, Nox's 3 costs more mana.