Yeah it's a common issue. The standard glass is, well, ground glass, while the newer plastic screens include a Fresnel lens design with concentric circles that spread the light across the whole area making the screen brighter and virtually vignetting-free.
However, glass is said to be easier to focus with, while the brighter the fresnel screen is, the harder it gets to achieve that 'snap' in focus, so it's a compromise. In my opinion, one of the best compromises is the mamiya rb/rz matte screen, but you have to cut it yourself.
