I voted for Voldemort, using the following logic:
- Force powers and magic are more or less equivalent
- Voldemort has magic
- Darth Vader has force powers AND a lightsaber
So, logically, Vader should be able to win.
On the other hand... I'm really not convinced by the idea of a lightsaber blocking a magic spell. Both force powers and spells work via the same kind of logic, ie. "It just happens!", and since a lightsaber can't block a force power, I don't think it could block a spell.
Also, if Voldemort is a master of Occlumency, then Vader may have trouble attacking at all, since Voldemort could feasibly foresee the attack and dodge. But, the Force can be used to play mind tricks too, so maybe it would be more even than we've supposed...
Also, one last thing: Voldemort needs his wand to use magic, but Vader doesn't need his lightsaber to use the Force; it's just handy to have. So if the fight ever got to the stage where they were disarming each other, Vader would have a considerable advantage, since if he could snatch Voldy's wand out of his hand with the force, Voldy would be pretty much defenceless.
Although, of course, if Voldemort has a stash of Horcruxes then he can't actually be killed
