Choose Headhunter when your collection is wheeled and tracked armour rather than Walkers, and when you expect to face Monsters and Vehicles β Knights, big Tyranid creatures, other armour lists β because both of its damage stratagems are written specifically against those targets.
The comparison people actually need is with Ironstorm Spearhead, the other 2-DP armour detachment. Ironstorm gives every unit in the army a free per-phase re-roll of one Hit, Wound or Damage roll, includes Dreadnoughts, and leans on Techmarine enhancements. Headhunter is narrower and sharper: only non-Walker Vehicles benefit, the payoff is a turn-by-turn choice between a guaranteed 6" Advance and a Damage re-roll, and its stratagems are pointed hard at big enemy models. Take Ironstorm for a broad, forgiving armour list with Dreadnoughts; take Headhunter for a mobile tank line into an armoured meta.
At 2 DP it leaves 1 DP spare at 2,000 points, so a Fulguris Speeder wing or Subversion Assets scoring package slots in nicely.
Sample gameplan: field four to six non-Walker vehicles and nominate your three best as Characters, carrying Gunnery Honours, Redoubtable Machine Spirit and, if you can afford it, Astartes Tank Ace. Early turns, Advance the flat 6" to claim firing angles without dice risk. Once positioned, stop Advancing so every tank has its Damage re-roll, and combine Target Weak Point with Kill Shot to delete the enemy's heaviest model. Answer incoming fire with Machine Vengeance, and use Rapid Gunnery to shoot your way out of anything that tries to tie a tank up in melee.