I have had BT Medium Shields that have held up very well. Conversely, I have Classic Mediums warp on my after an errant light pole hit. I thought they were both made of the same plastic, but now I think otherwise. Loved the feel of both, though.
As for straight Classic, in my experience it holds up well enough. Nearly on par with Pro or X plastic.
As for straight Classic, in my experience it holds up well enough. Nearly on par with Pro or X plastic.