Yes, glass filters generally are better than the plastic/resin. The Singh-Rah are glass.

The big difference between the glass and plastic is that the glass filters are true-ND while the plastic/resin (such as Cokin) are just a gray that is 'painted' on to the filter.

The resulting color cast (which is even apparent in a few glass filters, but only at extreme ND and very long exposure) is due to the plastic/resin filter's inability to reduce the amount of infrared that is transmitted, which builds up on the red sensor at varying degrees, starting at light pink and going all the way to purple. This is the color cast we see using the plastic/resin filters, even at short exposures.

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Michael A. Mariant
Nikonians Academy Faculty
Master Your Vision Workshops