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they both have a well functioning elder wand.

who wins and why?

Based on just movie feats it's hard to say, but I'd go with Grindelwald.

Grindelwald has by far the better feats due to power creep and SFX creep, but Voldemort is canonically Grindelwald's superior so he wins. Consider also that Dumbledore solo'd Grindelwald despite the latter having the Elder Wand and that Voldemort held his own against Dumbledore and more or less fought him to a draw despite the fact that Dumbledore had said wand.

Grindelwald.
Dumbledore looked superior in his fight with Voldemort. Voldemort did try his best to hold him briefly before running away as he clearly would have lost. Voldemort is not on Dumbledore's level.
And although I favour Dumbledore over Grindelwald, he's definitely on his level.

Originally posted by 9jaboy
Grindelwald.
Dumbledore looked superior in his fight with Voldemort. Voldemort did try his best to hold him briefly before running away as he clearly would have lost. Voldemort is not on Dumbledore's level.
And although I favour Dumbledore over Grindelwald, he's definitely on his level. Dumbledore and Voldemort stalemated while the former had the Elder Wand. Voldemort ran away, but that's literally because a team of aurors were coming to arrest him, hence this dialogue:

Dumbledore: It was foolish to come here tonight, Tom. The Aurors are on their way.
Voldemort: By that time I will be gone. And you will be dead.

These lines right before the fight tell us that Voldemort's contest with Dumbledore has a time limit. Voldemort at least believes he can ultimately prevail over Dumbledore. But he doesn't lack his chances taking on an entire strike team of Aurors and other powerful wizards while Dumbledore is still alive to assist in the fight.

Grindelwald straight up lost while having said Elder Wand.

If anyone is not on Dumbledore's level it is assuredly Grindelwald.