Well The wither is harder I think, Because he is alway in your face and attack you constantly, And the EnderDragon go away and come back few moment later. So The wither is Harder to kill, But still easy.
The Windows 8 Caracter! No, seriously, Something original... Maybe an series of Skin that go with a mod, mo'creatures, World edit, I know its hard to associate an images to a mod but... You can try!