According to Osho, Aum is a mild, soothing mantra that resonates from throat to heart and suits simple, heart-centered seekers, as in Patanjali's time. Hoo, distilled from "Allah," is forceful, strikes the hara/sex center, and better fits modern, head-and-sex-oriented people, propelling energy upward toward the heart. Begin with Hoo; when sex preoccupation fades and heart awakens, you may shift to Aum, though Hoo alone can complete the journey.
Use Hoo to lift energy from sex toward the heart; use Aum when your heart is already open and calm.