Whilst Moffat has famously added to the roster of classic Who monsters in the form of the Weeping Angels and the Silence, Russell T Davies hasn't been as prolific. Whilst Davies has introduced creatures like the Slitheen and the Sycorax, he prefers to use known monsters, such as the Daleks, the Master and even the Ood for the familiarity of his audience. This means that when writes a big episode, there's a big, known threat attacking the Doctor, and plenty of interplay between known enemies. Moffat on the other hand plays with the discovery of alien life, and lets the Doctor find out about his enemy as the episode progresses ('The Impossible Astronaut', 'The Time of Angels'). When Moffat uses the Daleks, the Cybermen, and even when he reuses his own creations, he adds to the mythology each time.
