OK, here is a source of contention that I hear about to no end. And here is my final belief on the pros and cons of plastic versus glass fermenters:
Like everything, there is a pro side and there is a con side. Since I teach my beginners to start fermenting in plastic, and then transfer to glass for a week or two of aging and cold-stabilizing, I will give the PRO side of fermenting in plastic containers.
Buckets (with tight-fitting lids and airlocks) have the following benefits of glass carboys for PRIMARY fermentation:
The CON side of buckets is that you should not scratch the plastic inside, or else you may harbor permanent infections, but eventually it is good to replace, or downgrade, your bucket anyway. (Make it a "sterilization bucket" after a few years of usage....)
Carboys never need replacing (unless you shatter them), but if you do want to ferment in all glass, I would recommend a 6 or 6.5-gallon carboy, and then invest in a wort-chiller, and have a five-gallon fermenter for secondary aging of the finished product. This is how some "more advanced" brewers will go, but you really WILL want that wort chiller if you plan on using the carboy-to-carboy method.
Obviously, this carboy method is for the brewer who has already dedicated himself to the hobby and is willing to invest in more equipment, spend more money on the hobby, and is also willing to deal with, logistically, more troubling scenarios. Either way, though, you can make excellent beer.
E-mail me with questions or comments on this matter - I am interested....