You need to do the maths. Your Hardin will obviously have higher Atk than Def because of his weapon, like most units (except ultra-tank units like Lukas), but that's not the end of the story.
Glimmer triggers based on the damage dealt after calculating the enemy's defences, not the raw Atk stat (that's the Dragon Fang/Draconic Aura set). So Glimmer's results can vary significantly, and can even result in 0 damage against extreme cases like Lukas who can exceed 53 Def.
Ignis, on the other hand, is 80% of the Def stat (36 in Hardin's case, before you add in Brazen Def/Res), which is a minimum 28 extra damage when it triggers, and isn't dependent on any of the enemy's stats.
Personally, I'd actually go for Bonfire over Ignis. With Bonfire, against an enemy who can counter, it's guaranteed to go off on the second hit, resulting in an almost certain kill since it's 18 extra damage added on to whatever you were already doing. Ignis isn't a bad option either, though, since you can just save it for the next round of combat, where it'll trigger on the first hit.