If you like a stiffer typing experience, you can choose Brass, especially with Top Mount. Brass tends to also provide a higher pitch tone. Flex-cuts have been added to reduce some of the stiffness of the plate.
Polycarbonate is a fairly strong plastic. Because of this, we removed flex-cuts in favor of having a fuller sound. With Polycarbonate, you can get a soft typing experience, and fairly deep sound.
Pom is a self-lubricating material, which is also relatively soft. Unlike Polycarbonate, the difference is that POM can give a deeper sound profile.
FR4 is a fiberglass material that is also used to make PCBs. With FR4, you can get hardness between Polycarbonate and Aluminum. It is fairly flexible and had a warm muted sound.