The Lapras with Ice Shard will do more damage before fainting. Moveset has almost always been more important than IVs in combat ability.
A difference of 5 HP, which is 10 HP on defenders does not make up for the higher damage and energy that Ice Shard gives over Frost Breath.
That being said, I think gym defence is irrelevant right now so it doesn't really matter, and should you power the Frost Breath one instead, it'll still make a good defender.