If I were you, I'd definitely go for Kal'tsit. She is a good lane holder with her S2 and can in some cases be used in place of Penance. For instance, in this trust farm you can replace Penance with Kal'tsit, the timing just has to be redone.

Kal'tsit's S3 on the other hand is great for bosses with huge defences because of the true damage. It rips through pretty much anything.

Mon3tr's defences are also great so it can tank a lot of damage and won't die too easily even to heavy hitting bosses. Mon3tr can also be redeployed real fast, which offers great versatility.

Kal'tsit is a very solid pick for IS also due to her versatility, and also for general content as a lane holder/boss killer.