Already have Herc. Go NP2 or Carmilla?

I'm F2P my 5* and 4* servants are
Vlad lvl 20 just got a couple days ago
Herc lvl 90
Sieg lvl 67
Emiya lvl 80
Iri lvl 80
Lancelot lvl 67
Since Herc is my go to damage for hard content atm. His NP2 seems like a good upgrade according to the numbers.
But my only assassins are Kojirou and Hassan cursed arm who I've only drawn once. So Carmilla is tempting.