From what I've seen, the 'Better Bottle' carboy is just a PET bottle with a brand name on it. I read that there are some PET carboys that have a special layer of oxygen impermeable material between the outer and inner layers making them ideal for fermentation. Are 'Better Bottles' among the PET carboys with this impermeable layer or something else?
Better Bottles use a superior manufacturing method to create practically impermeable PET plastic carboys
For more details, look under the "Permability" section of the technical page of their website.
This is anecdotal, but I have used PET/PETE water cooler bottles as fermentors side-by-side with Better Bottles, and have noticed no difference in oxidation rates between the two over four to six months. Both are marked with U.S. recycling symbol #1.
I believe the primary determinant of oxygen permeability with PET/PETE is its wall thickness, and I am not aware of any fermentor or water bottle manufacturer who is sandwiching any layer in the middle of the sidewall.
And, as Better Bottle states on their site, the oxygen permeating through the liquid in the airlock vastly outweighs the oxygen permeating through the plastic.
What may be manufactured better on a Better Bottle is the neck. Better Bottle makes a big deal about how close the tolerance is on their bottle necks, thereby allowing a near-perfect seal with their closure, relative to any glass carboy. I don't know if other PET bottle manufacturers achieve the same tolerances as Better Bottle.