Flame, Ash and Smoke Tactics
You have some straightforward tactical options, and some unusual bits you may want to add. You have dwarfs, strong artillery, monstrous cavalry, monsters/daemons and cheap infantry. Potentially with no synergy, but to find one, you’ll have to look deeper. So, below you’d find some tactical bits and pieces.
Infernal Guard (core) or Ironsworn (special).
Infernal Guard would fill Core requirements, and their single model un-upgraded is cheaper - 12 vs 17 pts. Ironsworn count against Special and you basically can’t upgrade them (can add up to 50 pts banner).
Talking about stats: The first difference is in WS 4 vs 5, but your basic WS 4 will be enough in most cases. If in doubts check your To Hit table.
Next difference is in S - 4 vs 5 (that is with enscrolled weapons counted towards). You can upgrade your Infernals’ strength by adding great weapons or fireglaives (and by all means, you will do that) which you cannot do with Ironsworn.
With Great Weapons, Infernals will have S6 vs S5 Ironsworn, but cost per model would be raised to 15 pts vs 17. Initiative won’t matter much, unless you’re playing against Dwarfs or other low I armies. And yours will be 2 or less anyway.
In second case (fireglaive) it will be S5 vs S5, and point value 17 vs 17 per each model, but Infernals will give you nice output in shooting phase, and Ironsworn will remain with better armour save.
Ironsworn should be more comparable to Dwarfen Ironbreakers. Almost same stats, same Armour Save. But Ironsworn have S5, and cost you 17 pts and Ironbreakers S4 and 14 pts. Ironsworn also have flaming attacks ward save 5++, and their Enscrolle Weapons can be used for Parry Save (6++). So I’d say they have potential, if you’re looking for damage delivering tarpit. With AS 3+, T4, S5, they will be able to hold and crumble most of the core or special infantry units in the game. I strongly suggest adding a Castellan to ensure their stubbornness.
Verdict: Infernal Guard for offence, Ironsworn for tarpit.
Overall, this comparison may be moot after all. In any army under 3k you don’t have points to splash out on Ironsworn. Infernals are core, and you should take exactly 25% core on the nose. you’ve got much better things to spend your points on than Ironsworn