If I were you, I would try double daggers and a bow,
here is a list of skills for a thief.
Leaping Death Blossom is the coolest weapon combo skill for a Thief (IMHO) coupled with the fact that daggers seem to be the strongest melee weapon for thieves. As for your secondary weapon set, I would choose a ranged weapon for more versatility. Pistols are definitely higher single target damage, but bows dominate at AoE for thieves.
Depending on what you prefer tho, you could go Pistol/Pistol instead of Dagger/Dagger. Sword is only Main-Hand and seems to be more geared towards PvP.
Please note, this only applies to what we have seen
so far as skills may be changed out, or revised, and we also have very little info about Traits for most classes.