At the same price, glass will resist scratches better than plastic, and feel more like the device's screen, though higher priced plastic protectors may be better than the cheapest tempered glass models, since glass will shatter or crack with sufficient impact force.
At an equivalent cost, glass provides superior scratch resistance and a more realistic touch experience than plastic, though higher-end plastic shields might surpass the cheapest tempered glass models due to glass's tendency to shatter under forceful impacts.