For PvP I prefer a Warlock running Voidwalker. I chose this because whenever I run Sunsinger, or see someone running Sunsinger who pops Radiance, there is always some dipshit waiting to kill the Warlock with a shotgun blast to the face! It is much more satisfying droppin purple death bombs on unsuspecting foes. For PvE, I run Sunsinger mainly because I find that having that resurrection ability is extremely helpful in a plethora of situations. Granted, in low level situations it usually isn't needed. However, every once and while a situation pops up where it may save you from a wipe. In many boss fight situations, I will run Voidwalker, use my Nova Bomb, then switch back to Sunsinger in case a second life may be needed.