Can you change Morinth at all in ME2 if you recruit her? Samara's mission seems like it was written to give some sort of sympathy to the Ardat Yakshi. They're described in a manner similar to vampires, and imprisoned for what they are; however I can find no redeeming quality to Morinth when she is finally encountered at all. Other than the lulz, I can't tell what the point of the choosing between Samara and Morinth is.
There is no redemption. Some beings are beyond any redemption, with no wants but to kill for the thrill of killing, by itself.
If you use her, it's not to change her. It's to have her as a tool, to facilitate your fight. In the RP sense, while Morinth may be a more dangerous being... well, she's a more dangerous being - whereas Samara is both tied down and unpredictable due to her code. You know exactly where you stand with Morinth, and can execute her if necessary.
My RacistShep will pick Morinth unless he doesn't pass the persuasion check (I'll go with the moment). He just wants powerful aliens. He won't reject them, but he won't love em. If Morinth claims to be a genetic destiny and can control organic minds, that RacistShep will consider that to be more useful when, you know, fighting the organic-formed Collectors (they are made of tech, but artificial organic tech). Whatever happens afterward is not the concern.
I believe Morinth IS outright 'stronger' than Samara overall, so that's what the decision is about.