Depends on your build really. But assuming you are level 15+ with high focus on agility, some strength + all fighter's guild advanced skills maxed out any 13speed 7%crit weapon beats Sonnet.
So for rogues the Bluesteel Poisonous dagger is best, Cleric and Warrior have to make due with a normal Bluesteel Dagger at the moment.
Higher speed and 3% more crit makes all the difference as it increases the likelyhood of instant kills with from massive criticals aswell as the overall damage on enemies immune to instant kills just from much better armor mitigation on crit.
Also Duel ramps up faster and you maximize the effectiveness of 15 extra damage from Heavyhand and Duel.
For rogues without fighter's guild access the crit bonus and dps on Bluesteel dagger still beats Sonnet on most fights - I'd say Sonnet becomes better on heavily armoured undead enemies though.
Best light weapon for trolls depends on your level. With very high stats the fire dagger can actually beat magma shortsword, especially on warrior. Massive criticals instant kills on cleave attacks are just too good =)