You can really scrub glass, whereas plastic can get scratched, and nasties can hide in the scratches.
Glass carboys can break; just a little googling will find you horror stories about broken glass, shredded flesh, multiple stitches, nerve damage.
I have one glass fermenter, and I prefer my plastic. Other brewers swear by glass. Not me.
Do yourself a favor though and get a hydrometer. The question always comes up, "How do I know when fermentation is complete?" Other than, "just wait" a hydrometer is the only way to know for sure.