This might be a bit long but i think will give you some good advice:
First of all, this is personal preference, so you either have to give a lot more information about your game style, or we cannot really recomend one.
Metal (aluminium) are used for high DPI settings. It has very low friction so you need a lighter mice or it will recoil your movments (g500 is not light, medium at best).
This form is usually used for finger or finger-wrist grips (claw grip). This is becouse your fingers cant move the mice far away on their own but have very high accuracy.
Non-metal (nano fiber, etc). This is more for a wrist-arm grip. Since you will move the mice with your whole arm, you want it to require some force to move it. This means you need to lower DPI so that the accuracy dosent go off the window.
That is the "General" information. After this comes your personal preference.
EDIT: As my personal preference i like BIG pads. Main reason is to keep my arm on the same hight as my hand, so i dont have even a slight "arc" between my fingers and elbow (this incurs in wrist pain over time).