So Nightstalker has the AWESOME perk of keen scout. Seriously though, I love this perk to death because knowing where your enemy is after being tagged is invaluable.
What plagues Nightstalker is the fact that its super is quite "meh" in the crucible.
As far as gunslinger, it's a slayer, there's no real disadvantage besides YOU DON'T GET KEEN SCOUT, but other than that you're fine.
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Nightstalkers keen scout has made it possible for me to literally go around a corner and shoot a guy where i wouldnt be able to any other way because i dont know where he is