Mata Hari or Stheno?

Stheno being a 4* means that her stats are much, much higher, and she additionally has a skill locked behind a strengthening quest that makes her (and potentially your other team members) hit significantly harder, making her probably the better choice - though this really depends rather significantly on what your four leveled Assassins are. Mata Hari does have several useful effects on her skills and produces a decent amount of crit stars, but her stats are as low as you'd expect from a 1* Assassin.

I have never played with matta Hari, but I do have a leveled steno. Honestly the only reason I did level her was to be a stat stick since I rolled her so early. The only time her np has ever killed anything was against centaurs, otherwise it is just a defense down. I found the lack of damage in the np very aggravating as a new player. The third skill after rankup did make her more appealing to use in summer event when she was a bonus for certain drops. In fact event bonus drops is the only reason that I pull her out, well that and to complete the gorgon family when I feel like memeing. I usually pick kojirou over steno if jack and shiki need backup.

Depens on how many divine servants you have. Mata hari, though weak, helped me kill goetia though.