With that being said i believe Multistrike and crit be best for Assasiantion mostly due to our big damage isnt really from poisons and since Mastery is so low because of gear not worth stacking (if you could even stack it). Im at 20 percent Multi right now with full honor gear and testing multi enchants. But with rupture ticks and venomous wounds being able to mutlistrike it gives back alot of energy and makes up for the low mastery poison. Multistrike envenom, DFA and Dispatch is really really noticeable.
I think that is why they buffed the effectiveness from mastery also for combat, because they were removing alot of it. Not sure how it works in PVE though
would really like to see numbers on this. mastery scaling is great for assassination but in a lot of cases the poison damage isn't relevant enough