I use a Braun professional care toothbrush, where the head vibrates and moves. Most dentists will reccomend such brushes.
Is the Sonic like a regular manual brush, but it vibrates?
All electric toothbrushes should be used without applying any pressure. The brush should be held lightly without pushing or scrubbing, like your friend suggests, it should glide over the teeth and gums.
I've never heard that electric brushes loosen fillings, but if you're concerned, you should ask your Dentist or Hygienist.