I believe that some prefer rock, I have a gigas 'chowder' and he loves hanging on the top of my live rock, even though he is as big or bigger than some of my pieces of live rock. I just got him too! yesturday I got him from kevinpo (A very experienced reefer) he has the tank of the month on some reef magazine!
I am owning a clam that was owned by a supastar! HEHE