Nightstalker brings alot of support to a fireteam, and tether is a great way to make orbs and increase dps on a boss. Plus I think bloodbound and a rocket launcher is the highest damaging aoe possible in game if mobs are bunched up enough. Stormcaller doesn't really offer any group synergy, in fact it's really a self supporting tree. Stormtrance is great for packs of mobs and you can do pretty good single target dps too, especially if you open with landfall. I think when it comes to supporting your fireteam sunsinger is more comparable to nightstalker, but not as versatile.