Well, Superman can fly, Hulk can't. That means Superman will always win, imply because he can do the one thing that always works against the Hulk, remove him from the battlefield. I don't think either of them can kill the other, but Superman has options Hulk does not. That or he could just kiss Hulk and make him forget why he's angry in the first place. Seriously though, why does Superman have a mind erasing kiss?