Yes, the new Mistresses are stronger when invisible and have increased stats. Also, the old Mistresses deal half damage at distances greater than half their range (like most ranged units) and have a penalty in melee combat (again, like most ranged units). But these new ones, along with Stalkers, tear through shooting/spellcasting neutral creatures like nothing. Although, against races with high attack but not very high defense, it's better to use the old Mistresses; otherwise, they'll be destroyed in melee combat. But against neutral creatures, the new ones are definitely better, and in the final stages, it depends on the situation.