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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I voted gunslinger. Night stalker has some good perks like smoke and shade step but lacks a good grenade and the super is situational. I've been wrecked by people who are good with it though. They are both outclassed by sun breaker though ha-ha;)