Hard rubber or soft rubber?
Rubbers can be made in different levels of softness / hardness. Soft rubbers tend to get a little less flexible / pliable in the cold weather while hard rubbers get softer in the warmth of summertime.
Therefore, by using soft rubbers in wintertime and hard rubbers in summertime you can even-out the textures of your squeegee blades across the year.
For very small squeegee there can be a tendency to use too much force to press them onto the glass, so hard rubbers are useful to withstand this pressure. Conversley, soft rubbers are helpful with larger squeegee blades where the pressure required is greater.