Toure has much more pace and is much better on the ball, most technically gifted central defender who isn't Brazillian and would give the likes of Lucio a run for his money. Don't forget Arsenal haven't conceded a goal in 7 games in europe and he's played against the likes of Ronaldo, Raul, Robinho, Trezeguet and Ibrahimovic, twice in some cases. All Terry has to do is mop up anything that gets past Makele whereas Arsenal are much more gung ho with our full backs bombing forward...
Plus Terry has the best keeper in europe if not the world behind him to help him out. If Terry pushed up more and someone with pace got in behind Terry would never catch up.