To lay it out simply...
Gunslingers have the superior DPS by far, and can put out extreme amounts of damage if played right, sadly they lack a lot of mobility.
Scoundrels, which have a stealth mechanism, are mobile and very good at burst DPS. The Scoundrel class, if played right, can stun lock their opponent and put out crazy bust DPS, killing the target ASAP. Sadly, however, they lack the sustainable DPS of a Gunslinger.