IMO Asajj lead is outdated(like Vader's). It's not terrible, but there are so many teams and individual toons with high tenacity, that the TM reduction gets reduced and once you get past the Zombie's 2 resses and Daka, it's over for that team.
With Talzin, you can wipe 3-4 members of their a NS team out, and they still have a chance to bring back the whole squad in a nasty chain of events, especially with plague going around. This is why she's preferred in PvP. Both leads give +30 speed, but the extra potency increases chance for plague, and the chance to your entire team to come back at any given time can be very punishing if the enemy isn't careful.
Asajj's lead is best for p4 in sith raid, when Nihilus is still in play because the TM reduction on enemy + TM gain for NS due to how any time you take an action, you take damage.
