Depends on the map for me. In tight spaces sunsinger is better because the grenades murder, and it is harder to kill a radiant warlock in close quarters. Meanwhile voidwalker has a harder time effectively blinking and nova bombs tend to hit walls, doors, and ceilings a lot. On the other hand in the open voidwalkers can blink around quickly, and can use their novas very well while sunsingers have a harder time landing grenade hits.