The fighter with a better grasp of ma ai, or the Korean equivalent.
I did Kendo for about a year and am currently in Gumdo and while Kendo was more oriented towards the sport sparring aspect where I trained and the Gumdo is geared towards "battlefield" application, there really are only so many ways you can swing a curved, single edged sword - be it mok gum or bokuto. It's akin to saying "Who would win, a judoka or a jujutsuka?"
Like cxt said, they are essentially the same art. If you wanted to split hairs you could say that a kendoka would only have a shinai while a gumdo player would have a katana, but Musashi already showed us that stick can beat steel.