DF has an advantage or two....
first is a sleaker profile witn no engine at the front. second is the ducting acts like winglets for the *prop* this allows it to spin much faster giving a greater thrust than equlivent size 2 blade prop. the thrust is concentrated from the outlet and pushes better than the bigger wash from a prop. like having a shower head on centre feed compaired to lots of little holes at the edge.
* as i see it anyway *