In March 2011, I tested the "taffeta" versions of the same fabric called Intrepid, TG Intrepid, TH Momentum 90 T, & Quarktex (20d) taffeta by various vendors. The fabric from each vendor tested identical with an air permeability of 4.7 CFM and a HH of 949.04.
In March 2011, I also tested the "multiple ripstop" versions of the same fabric called calendared + DWR TG Intrepid, TH Momentum 90 MR (V2), & Quarktex (20d) mini-ripstop. They tested identical with an air permeability of 4.7 CFM and a HH of 559.6
As a general rule, ripstop fabrics have a very significant increase in abrasion resistance (2-4x) but, less HH than the equivalent weave taffeta versions. The silicone coating knife or calendaring process seems to create a less uniform surface when it encounters the larger diameter ripstop threads as opposed to the uniform surface taffeta.
See submissions 14 and 15 at http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=45026&disable_pagination=1