For crystal cases, the plastic quality isn't an issue as it will crack if you drop it anyway but it might have bad molding. I had one where I had difficulty opening my 3ds as it squeezed the hinges of the 3ds and rubbed some of the metallic paint off the hinge corner... I ended up just using the bottom case and put an exterior screen protector instead. Anyway, I liked the way it looked without the case.

TPU cases are a soft rubbery type of plastic.