This was debated in the 10-19 bracket and here was what I found.
Shadowfang gets 6 poision procs and 1 shadow bolt proc in 54 seconds
Goblin Screwdriver gets 9 poison procs in 54 seconds
WsG dagger gets 9.5 poison procs in 54 seconds
The 1 agility provides a bigger ambush and a higher average DPS than that 0.5 poison proc
EDIT: also your gearset is flawed, you are giving up 10 crit 6 stam for 2 agility on shoulders but you prefer 6 stamina 11 crit over 4 agility on your chest, either use loom chest or use armor of the fang (I am assuming you can't get gloom). Same thing applies for your legs, you may aswell use 'of agility' legs with 5 agility 3 crit enchant since its 4 agility vs 13 stamina 7 crit 6 haste.