
Max mana on Assasin is 252 (312 with nile)
If i play with assa i prefer playing with Nile. So i have 312 mana.
Using plague you can toss roughly 4 times. 4 x 500 dmg = 2k dmg over radius 9
Using Milk you can toss 2 times, almost 3. So lets say 2.5 x 900 dmg = 2.25k dmg over radius 5
Doing events like skull and cube you can use plagues because of the radius (more targets). Some ppl die because of the minions instead of the boss

Doing events like pent and lord i prefe Milk (single target)
Imo it all depends if you are in a busy realm with a lot of event farmers because you want to deal a lot of dmg in the amount of time you have.
So i think Milk>Plague during events in a busy realm.
When doing Oryx1 and 2, it all depends how skilled you are. Plague for safe playing and Milk for the experienced players.
In case of mana ( if you dont have any pots or something like that, low mana) plague is better.
Something like this
