Why not try both AD and CDR? Oh, and while you're at it...get some Apen to go with it?
In my opinion, Riven should run AD Marks and Quints, Armor Seals, and CDR Glyphs (Flats if you have em, per level if not). Then first damage item you should be rushing is Brutalizer...AD, Apen, and CDR all rolled into one lovely item that Riven can truly abuse.
After that, Stacking raw AD, ARM, and MR is a great way to enhance her survival at the same time you are boosting her damage output. HP helps too, since the more HP you have, the "longer" you have to pull off more shields (takes enemies longer to kill you, possibly granting you one last shield before you die).
Taking lifesteal is also a great thing to do with Riven so she can sustain. Nothing at all wrong with CDR boots, 4x Dblades, and a Brutalizer due to a poor early game. Not the most ideal, but gives Riven a little of everything she really wants...