Clear body is probably the better choice. Light metal is more situational, that is, it'll only be useful with weight based moves. If you plan on using any with Metagross, then it would be a good idea to look into it, but stat loss prevention will help you more. (Though even that is somewhat situational on PO) There aren't many weight based moves, Body purge, heavy slam, heat stamp, grass knot, and low kick are the only moves I know of that concern weight. The ones metagross would be most concerned about are heat stamp and low kick. Low kick is neutral against him, and thanks to his LM, it would get weaker. Heat stamp is super effective, but rarely comes up. His high defenses would also make these moves just bounce off. Clear body is nice for blocking out stat drops that might come up. Intimidate still pops up from time to time.