I don't think that one is stronger than the other. I think that it is determined by light and flow. Zoas, 'shrooms and Rics often come from the same habitat - they often come in on the same rock. The dominant species is determined by environmental conditions. Increase light and/or flow and you tip the scales towards most zoas. Allow a mushroom to inflate fully in calmer waters and it will shade the neighboring zoas, inhibiting their growth.