I'm sure you will get about a 50-50 response on this. I will offer up my opinion. I choose glass for 2 reasons. The first is cleanliness/sanitation. Plastic buckets can scratch easily, which gives bacteria a place to hide. With glass, I don't have to worry about it getting scratched. Second, I like having the ability to watch the fermentation process. With a bucket I would have to remove the lid any time I wanted to check on the progress. With glass, simply lift the towel off the carboy. Its really cool to watch fermentation take place.
Having said all that, the biggest reason people will choose plastic is safety. If you drop a glass carboy, you may as well dial 911 before it hits the floor.