My alt character is a 27 warlock. I use a maxed out voidfang. I find it to be very good for both subclasses. Spawning with grenade energy is very helpful for both classes. As a sunsinger, you can spawn with 2 grenades which is great for crucible or a fresh revive. As a voidwalker, the enhanced axion bolt is also very helpful. I do not own the heart of the praxic fire. To me, I'm nor sure how useful the increased cooldowns would be because I only use radiance for when I go down, and after the revive I usually just run and try not to die again rather than spamming grenades. Heart of praxic fire is only helpful for sunsingers. In the end, I would say get both and max them. They are both great chest pieces