'Petrified' means "to be so scared that you're turned into a stone." Petrus = Greek word for 'rock.' And, of course, in Harry Potter 'petrify' literally means "to turn to stone." :) So, 'to be petrified' is being more scared than 'being terrified.'

I vote for petrified.

Although both words can be used synonymously, I'm not sure how one can show more fear than the other. It's not a defined scale of fear, you understand; just different words describing the same thing from different angles. Look at the meanings of the words themselves: 'terrified' of course means you're a victim of, or are filled with, terror. 'Petrified' is more figurative: you are so scared you are 'turned to stone', ie. cannot move. From the Greek for 'stone', "petros".