The Better-Bottles specifically, and PET plastic in general, has negligible oxygen permeability. Many believe that glass carboys have a zero oxygen permeability. Not so. While it is true that the glass itself will not allow any gas exchange, the same cannot be said for the stopper, which is gum rubber or vinyl. So the overall oxygen permeability is not zero.
I don't know the hard numbers between the oxygen permeability of glass carboys with rubber/ vinyl stoppers and Better-Bottles with their PET ported closures and the other PET accessories, but I'd be willing to bet there is not any significant difference.