Metallic plastic and strong and flexible plastic are both made with nylon / polyamide. However (I'm quoting the materials page here), "Metallic Plastic is Nylon Plastic filled with Aluminum dust. This dust gives Metallic Plastic the sparkle, but also makes the material more brittle than the Strong & Flexible material." I've never tried metallic plastic, so I con't know how much more brittle is it compared to classic S&F plastic.
On the first version, the two sides could be bent to the point they touch each other, without breaking. The first version was a bit loose, so I've made the sides closer and thicker, they should fit more tightly on the head.