Honestly with little idea of what's to come in terms of further balance and subclasses, and little idea of how some of these deeper skills will work, you should probably just abide by the classic RPG adage of picking whichever one you think looks the coolest.
Also some stuff on Sunsinger...
Sunsinger is a level 15 subclass in Destiny used by Warlocks. Sunsinger's super ability, Radiance, will revive fallen Guardians and boost the effectiveness of all of your and your allies' abilities. Sunsinger has been described as a team-oriented, support-based focus that includes some area-control abilities that allow players to block pathways or doors and keep enemies from getting close.
Sounds useful but not very Warlock'ish.
Makes sense to me. The warlock is the mage class, and mage classes typically spec towards either elemental DPS or healer/support. Sunslinger is a healer.