QSAR - new question on chirality descriptors

Dear,
   I should appreciate if you answer me the following question:
   In order to do a QSAR on chiral compounds (e.g. barbiturates) and to
include chirality, which chiral descriptor seems more interesting:
   a) experimental (+)/(-) deviation of the polarization plane, or
   b) "geometric" (R)/(S) IUPAC (Cahn-Ingold-Prelog) structure?
   Thank you for your attention,
                 yours sincerely,

-- 
Prof. Dr. Francisco Torrens
Institut Universitari de Ciencia Molecular
Departament de Quimica Fisica
Universitat de Valencia
Dr. Moliner-50
E-46100 Burjassot (Valencia)
Spain
Telephone: 34.963543182
Fax: 34.963543156
E-mail: Francisco.Torrens-*-uv.es