Comparing Snipers to Sleeper isn't that great. Snipers require a head shot to kill in one hit and can't kill someone in their super even with a head shot. Sleeper can kill a full health Guardian in their super with a body shot from across the map and you get enough ammo for a team wipe from the pickup in the middle. Snipers can wreak some havoc in a skilled hand, but Sleeper is very low risk and very high reward in Gambit.