Here is a good thread on the subject with links to even more threads. Basically it boils down to the type of plastic used.
Another issue is that if you are going to be fermenting in a 5 gallon jug, your batch size will actually need to be closer to 4G to avoid excessive blow-off. Most people using Better Bottles get the larger size for the fermenter. Other options for fermenting include buckets and glass carboys, each with pros and cons. http://www.homebrewtalk.com/f25/water-cooler-5-gallon-jug-primary-98560/