It all comes down to your style of play really.
Quiet Knife is built for burst DPS and the goal is to deal with your opponent before he can deal with you.
For that reason, the QK built is also more squishy than a gambler built, since you focus on damage, you sacrifice survivability.
Gambler on the other hand is the opposite. You play "cat & mouse" with your enemy and survivability is much higher.
Mischief I have no idea what it is about, nor will I never know....just one word of advice, avoid using it in the moors.
For the moors it is useful to trait Enrage if you are caught in the midst of several Black Arrows (they will hate you for traiting it).
You can trait DITE (Dust In The Eyes) for those situations where you have large warg-packs lurking about.
If you are standing face-to-toe with lots of Defilers, then you can make use of Subtle Stab, using the barter relic from Glân Vraig which reduces their healing.
As a moors-oriented burglar, it is important that you squeeze the most out of your little hobbit, since you will need to get used to facing opponents twice your size, with twice (or even three times) as much as morale as you have.
Most of it comes down to your rotations. Of course traits and skills matter, but if you have your rotations down and working, it doesn't matter if you are a Gambler, or a Quiet Knife addict - you are unstoppable (until they stop you).
One of the best ways to improve on your skills is to study other burglars and learn from others. Pick bits and bobs of their technique, combine it with your own and perfect your style.
If you are going for a Gambler build, look at Palesink, he is (in my opinion) one of the best Gamblers I've ever seen.