This isn't a contest at all; Sauron has overwhelming numerical superiority. If you erased Voldemort's personality and just kept his abilities he could win, but as it is his anti-muggle stance prevents him from capitalising on his world's superior technology.

Voldemort + Death Eaters + Aurors is a fairly formidadable force; between Apparition and Killing Curses they could pull of an effective guerilla war. But Sauron's upper leadership (himself + Nazgul) have some powerful magic themselves, and are sufficiently unkillable for a decapitation strategy to be implausable. They could probably do quite a number on the orc hordes with Dementors, and Potter-verse giants would appear to be superior to LotR trolls, but in the end numbers are likely to win out.