Madonna- her fashion sense has always come from an intellectual sensibility. From a point of view of someone who is interesting on the inside as well on the outside. That is why fashion designers the world over have been drawn to her, as well as she to them. All her looks from 80's street to sex siren to couture, Madonna works it better than anyone around.
As for Britney...well, she looked better in her mall fashions in the 90's then today. Designers are not chasing after to get her in their clothes. They know she has a way of making anything look instantly cheap. Really, stylists can only do so much.