Grindelwald V.S. Voldemort?

A friend of mine insist that, if you were to pit the two of these together during their respective peaks Grindelwald, would defeat Voldemort in a wizard's duel. He backs up his claim with the fact that Dumbledore admitted himself that had just a little bit more skill then Grindelwald. While Voldemort however runs away every time he faces Dumbledore. What do you think?