Wizards are quite ignorant of the outside world, even ones with a fannish interest in Muggles like Mr. Weasley, and Voldemort and crew are extremely arrogant. Moreover, the Death Eaters suffer the problem of not making alliances well, being picky even about which *wizards* they will accept, basically giving the Muggles allies who have an understanding and use of magic.
Given that the Muggles have numbers, word of God says that a shotgun toting Muggle wins, very strong potential for allies with magic, and the fact that Death Eaters are about as subtle as week old road-kill and have arrogance that can be measured in kilo-saurons, I'm going to hand it to Muggles.