It depends on what you do in Destiny. For PvP, Voidfang is better if you constantly die due to the instant grenade charge. But for PvE, Heart of the Praxxic Fire is way better, especially for raiding. Reduced cooldowns allow you to throw grenades as soon as the animation is finished, with no down-time. And in PvP, an endless barrage of grenades is quite potent, don't you agree?