According to Osho, neither is better in isolation: AUM unfolds in stages. Begin with loud, whole-body chanting for months until every cell vibrates—this lays the foundation and gathers energy. Only then move to subtler stages, letting the sound grow softer and finally hearing it within. Inner listening is the culmination, but it becomes authentic and powerful only after the body is fully saturated.
First chant AUM out loud until your whole body hums; then it naturally turns inward and you simply listen inside.