I mean that's true for something like goggles and walls and such, but when getting hit, I don't understand how it could hurt more. something with the same weight size etc, will hurt the same imo, unless the bb deforms or shatters when it hits your skin, but right now I'm thinking that the bb is so much harder than your skin, that it will be basically the same.
In physics, (last semester) we learned about conservation of momentum, where an object bouncing off imparts the same force on the wall, but the net change in momentum in doubled. (totally elastic collision).
Density of the plastic bb and the glass bb will be the same, as density is mass over volume, and mass and volume are the same. You're thinking it will impart more energy upon impact because it doesn't deform (that is true), but I'm talking about getting hit (you as in person).