glass, i think, is cheaper. But it can be damaged easily. so if its going to get people throwing stones at it etc or is in a very windy location get plastic.
plastic though, gets scratched very easily, it also doesnt have the same properties as glass, It doesnt trap heat aswell. and heat that is in the greenhosue is lost quicker than through glass. Over time it can become cloudy andd then wont let very much light in.
You can buy special stuff that is designed to ahve the properties of glass but with the advantages of plastic, but tis very expensive,
