QRD vs BBC Skyline Diffusers

In your READ THIS FIRST (please!) thread under Acoustic Basics  you state that "The best type of diffuser is called a QRD..."  If I may challenge you here, and if this is true, how much better are these than BBC-style "skyline" quadratic diffusers and why?

I know that your Real Traps QRD diffusers have absorption behind the reflectors.  This is an improvement beyond simple diffusion, of course.  I'm speaking of generic quadratic residue diffusers here.  

Thanks, 

Michael