It depends on which version of Ares this is. If it's Marvel Ares or the one from God of War, then Grimmjow nails him into the ground and brutalizes him. If this is DC Ares who just seems to get more powerful the more you fight, then I don't see Grimmjow winning. As far as I've seen, Marvel Ares is not all that fast. He's certainly stronger than Grimmjow, physically, but in the aspect of soeed, Grimmjow completely dominates him. For those that have seen Bleach, you'd know that Ichigo is insanely fast in his Bankai state. You'd also know that when Grimmjow and Ichigo first fought, Grimmjow utterly mopped the floor with Bankai Ichigo with almost no effort.
Marvel Ares can be one tough mo-fo, but against someone like Grimmjow, he'd be lucky to walk away in defeat. Grimmjow is already quite powerful. In his released state he's a match for Vizard Ichigo.