It's not much of a difference tbh. I think part of that is remenant belief from Diablo II! Fairly sure blizz have stated they didn't want it to be a big difference or people would just farm the cow level and no other parts of the game. And that is not what they want.
According to the theories in forums, the secret cow level seems to drop best loot in the game (one step above last level of your difficulty), and also has significantly higher droprate of legendaries.