Ibaraki Douji vs Herc

Ibaraki and Heracles are very similar (at higher skill levels, Heracles performs better at low skill levels). The one major difference is hat Ibaraki has a lot more utility (buff removal, debuff cleansing, charisma-variant, etc.) in return for a lack of hard survivability tools such as evasion or guts.

Normally I'd suggest that you level someone else but there are 2 main things to consider:

1. You have max bonded Heracles. Fielding him means wasting bond points. Even if that is a bit minor in the long run, people really dislike wastage. Ibaraki can replace him suitably, allowing you to still get bond points.
2. A heavy Berserker like Ibaraki is always a useful addition for hybrid nodes, the upcoming Shunjuku singularity (bonus for evil Servants), and whenever you feel lazy and can't be bothered to adjust a composition for a particular quests. She is also a bonus Servant surprisingly often, but not for a while yet.

Ibaraki and Herc may look similar but the truth is the way they are used are completely different.

1. Ibaraki
Ibaraki is a frontline berserker. She is best used as your main source of damage. Just look at her skillset. First skill is a supportive skill and allows her NP damage to be higher than Herc's. Second skill makes her performance to be more consistent since she can remove the debuff she got. Third skill is enough for survival skill since the defense buff is massive for 1 turn and with good support she won't die that easily. Not only that, she is better at filling her NP gauge than Herc due to her better hit count in both quick and extra attack.

2. Heracles
Herc is best used as a backline berserker aka secondary damage dealer aka last man standing. Putting him in front with his bond ce can result in you wasting his 3 guts since there's a chance that your enemy would hit him three times in a single turn whereas as a last man, he is sure to be hit only once every turn allowing you to have more turns before Herc finally dead. His attack stats is higher than Ibaraki. As a last man standing, Herc can deal a massive amount of damage because of his buster brave chain whereas putting him in front would make his buster brave chain to be rarely happen. His performance is not as consistent as Ibaraki since if he got a debuff, well you're stuck with that debuff until the debuff gone by itself. I'm not saying that Herc is bad if you put him in front, but with the same support Ibaraki performance is clearly more consistent than Herc.