Personally, I have both. I have found that no matter which you use, get carboys carriers. Steve mentioned the plastic is flexible and can be squeezed, ejecting the bungs....been there done that and got soaked with wine. Bought a carrier after that. I intend to build the wooden crates mentioned as well, as I now have 10 carboys.
Glass, again, heavy and slippery as was mentioned. Didn't break any yet, but had some close calls. I find the absolute best is to clean the carboys right after they are emptied. This reduces the effort and potential scratching posed by plastic.