Here's the thing. Both of these guys are fast, if you are going to give Voldemort the apparition (sp?) we see from the other Death Eaters in OotP. I don't think he can move faster than the eye can see, and he certainly can't attack while flying around like that. He has to materialize first. Shane doesn't have this problem. Any spell that he can see, such as an AK for instance, can easily be dodged by that kind of speed, to say nothing of him simply appearing behind Voldemort and ending the fight right there. It seems to me, you see, that the HP characters have two main weaknesses: speed, particularly in relation to melee attacks, and durability. In both of those areas, Voldy is outclassed.