Ok, this is quite a good question. It depends on several factors, such as the shape of the glasses, and the specific design of it. Generally speaking, plastic glasses are much lighter than metal glasses, which could have less pressure on your face. And there of course would be less pressure mark on your face. Anyway, it depends on you, you could decide what is good for you. But I still recommend metal glasses, for they are much more durable and cool.
I personally think metal glasses are more comfortable than plastic glasses. Mental glasses tend to be lighter on the face than plastic glasses due to their thinner frames and less bulk. Furthermore, as plastic glasses usually do not have adjustable nose pads, they are more prone to sliding down the nose. Therefore, I prefer mental glasses.