Each of them has own features so choose the right one depending on your application.
A medium squeegee, 60-65 durometers, is for most general T-shirt printing. Choose another one if you prefer sharper or thicker deposit of ink.
A hard squeegee, 70-75 durometers, is used when you wish to make a sharp finish. It does not bend so much and can scrape ink on the screen sharply so it is suitable for detailed printings such as fine lines. Do not press too much otherwise deposit of ink will become too less.
A soft squeegee is recommended when you try to achieve a thinner deposit of ink. Holding a squeegee with at lower angle by taking advantage of its flexibility, you can acheive a thicker print.
Do not put too much pressure otherwise the edge will not function properly and excesive deposit of ink may cause smuding.”
Flexibility of blade depends on thickness of rubber but also thickness of the blade. A soft and thin blade bends the most. The more it bends the more pressure applied area expands and can achieve a thicker deposit of ink.