More physical certainly. But an opensides game isn't purely about physicality (though it certainly helps!). Hooper doesn't run with the ball near as much as O'Brien, as he is used far more in a "linking" role for the Wallabies (a role which he is very effective in). When he does run he is arguably even more effective than O'Brien (though he does run a bit wider of the breakdown). Both are excellent at the breakdown, and though Hooper is improving on defense I think O'Brien has an advantage in that regards (due to his increased physicality). I can certainly see why people would prefer O'Brien over Hooper - personally I don't think there is much between them and it would really depend on the style of openside you prefer...