Which Hardin to keep?

Keep the +atk Hardin. Without that, he won’t break 50 base atk. That and boosting his Def is considerably easier with Ward Armor being available on the 3 star Draug

+def-atk... +atk-def... man the joke is on you

Well I've used a lot of -atk or -def iv units earlier on when the bane is not ideal at all for the type or spread.. Hardin really is one of the best armored units in the game with his (usually) mixed defenses and his Gradivus lance and aggressively badass artwork by Daisuke Izuka, so I'd keep at least one copy. Since those two ivs are really just an inverse of each other and there is no superboon/bane in the atk or def, Its up to you for which stat you want to forfeit. You can fix it temporarily with an atk+3 or def+3 seal or use one of those Squad Ace stat seals to look cooler. Or if you pick +atk-def, you could inherit Fortress Def 3 from 4* Seth (?) for his A slot or use it as a seal. It'll lower his atk with lance to a neutral 50 and bring his def up to 38, close to the +def iv. He should be better this way, and if you get a better iv later on, just merge this into the new one

If you're not in a hurry to fodder, try playing with both for at least what it takes to level them up and see if you prefer to play more defensively or aggressive with him?