Pre-Flashpoint Ares was extremely powerful. Diana mostly beat him due to plot induced stupidity. J'onn might have a few more abilities than Diana but is roughly on par with her in terms of sheer power. Ares is fundamentally more powerful than either of them and has the magical ability to counter most of J'onn's psychic powers. However the New 52 Ares is somewhat of a mystery to me power-wise. The book seems to focus more on plot than character ability (love the Wonder Woman book by the way). So I think J'onn would have a shot against New 52 Ares based off of my own hunch.
But Pre-Flashpoint Ares wins 10/10. If he was fighting Fernus the Burning it would be more interesting.