from what i am told the harder 3.5 or 4 to 5 the reed the smaller the mouthpiece opening, the softer the reed like 2's and 3's are used when u have a bigger mouthpiece opening.
well the difference change be also on what mouthpiece your using. softer reeds seem to soak up more water while harder reeds have to be soaked a lot longer in lukewarm water.