I assumed Joe was better too, so after seeing your post I went back and checked last years stats as well. They averaged the roughly the same minutes and the same runs per metre. Joe was good for another 2 runs per game which brings his metres up, but his offloads were almost double. Aloiai is good for another 4 tackles a game, but he missed much fewer tackles and was signicantly more disciplined. Joe had the double the error and gave away more than double the penalties as well.
You could say Joe is better value for money, because we'll get him for Josh's current salary, whereas Manly will be paying double that.