Different methods produce different effects. Scientific research on mindfulness emphasises awareness and alertness. Research on Acem Meditation and other sound-based techniques focuses on the deep and spontaneous relaxation response triggered by meditation, inducing a wakefulness that comes naturally, not as a result of concerted effort. By loosening up bodily and mental tensions, Acem Meditation lays the ground for deeper changes in personality.
Mindfulness aims to help the individual change perspective by bringing the attention back to the present moment, away from memories and fantasies. Acem Meditation seeks to expand the present moment to include all spontaneous mental activity, thereby accessing and modifying unconscious patterns of experience and behaviour. A relaxed and open mind is more likely to let go of established ideas and habits and to embrace new perspectives.
The difference in object of meditation is significant. By directing the attention towards breath, body and other spontaneous elements, mindfulness techniques encourage a receptive frame of mind. In Acem Meditation, the gentle repetition of a neutral meditation sound combines such receptivity with an enhanced ability to act in more productive and less stressful ways.
Just as important is the difference in mental attitude. While mindfulness aims at reducing mind wandering, Acem Meditation fosters a free and accepting attitude towards thoughts, feelings and even drowsiness or sleep. This mental freedom helps to release emotional pressure and access creative resources. It reduces the tendency to become mired in worries and rumination, and increases stress tolerance, robustness and energy.
See Dr. Øyvind Ellingsen's blog post "Mindfulness and Acem Meditation" for further discussion.