Well, my son always goes with execution as he is an extreme perfectionist. He never wants to compete a skill unless he does it well every day in practice. While most boys will shimmy their weight a little on a giant, for instance, my son will come down(in practice, and actually, in competition also). But his teammate is another story. His routine is packed with difficulty, but he has execution errors everywhere! By the end of the season he usually can do it well without the execution errors. A third gymnast does more difficulty BECAUSE he doesn't do easy routines well. So, I guess my answer is that the coach learns the tendencies of the gymnast and goes with that.