I prefer plastic glasses rather than metal glasses. I will tell you my own story. I wear plastic glasses for more than 10 years. Meanwhile I have a classmate who wears the metal glasses. Now when we both take off the glasses, the shape of my eyes changes little but that of my classmate's eyes are transformed. Her eyes seem to extrude from the orbit as well as her noses appear two deep sag on the area of glasses pads. Besides, the metal glasses might make you suffer allergy. So I recommend you to choose the plastic glasses.
Generally speaking, mental makes you look mature and plastic makes you look younger and active. So, you should it clearly that what kind of feeling you want to show yourself to others. You can ask your friends for advice, or go to the shops and have both of them a fitting and you can find what kind of glasses frame you like best.