The first thing you have to decide is if you are going to use the chest protector inside the clothes or outside.
And then think about the level of protection you want to have.
Basically to greater protection greater weight and also less comfort.
The chest protector that are placed over the clothes are less tight to the body, so they offer you greater freedom of movement, but their protection is less, since they usually protect less area of the body and also in case of fall can move more than your position.
The inner chest protector can be of the vest type, that only protect you chest, back and clavicles and the so-called integral ones that are like a jacket stuck to the body with a multitude of protections. They are very hot in summer.
The material of all the chest protectors is usually a combination of fabric, foam and plastic, although lately there are many manufacturers that include the 3DS.