first the 2 guys are so ugly xD lol hmm i think darth sidius wins beacuse he has mais battle abilities and he can use the force with a one mover voldemort is without his wand. also i amor estrela wars so i chose darth sidius definitely :D
It's an interesting match-up because both are quite powerful in their own universe. Powerful in terms of intelligence, willpower, charisma, political influence, & knowledge of their respective dark arts.
Both of them have the ability to hurt you at a distance & both prefer to let someone else do the hurting for them, so as not to get their hands soiled or easily implicated for their wrong-doings. Unfortunately, neither of them have any qualms about taking a life, either.
However, of the five Harry Potter films that I've seen, none of them portrayed Voldemort (nor any other wizard that I can recall) possessing the ability to "sense" an enemy at the door. I could be wrong though, it's been awhile since I've watched any Harry Potter.
Even if Voldemort CAN sense his enemies like a Sith or Jedi can, Sidious still has a Galactic Republic/Empire catering to any & all his wishes.
If Voldemort somehow avoids any Republic/Imperial forces & successfully approaches the Sith Lord, Sidious can defend himself quite capably using his Dark Force powers. If necessary, Sidious can also engage Voldemort in close combat using a Light Saber.
Bottom line-Voldemort doesn't stand a chance... IMHO.