The choice in favor of Furies is obvious. This means that the Assassins will fire at full strength at least twice. And that's at least 900 poison damage + 50% physical damage. And the Mistresses will go invisible.

The Assassins will poison 2 stacks, I agree. And the Hunters will poison 1. But they will deal full physical damage. And that will be more than an extra 450 units of poison damage, moreover, spread out over time.
And don't forget - you are going to poison high-level stacks, which means the physical damage will be very low. And with Hunters, you can get to low-level ranged units, and the damage to them will be many times higher. And the enemy will also remain on their territory to be finished off.