Samara vs Morinth, benefits of each choice?

Picking Morinth over Samara will give you the Dominate bonus power. Picking Samara will give you access to Reave.

Additionally, after the suicide mission, Shepard can take Morinth up on her offer, which does result in a game over.

If you are planning to continue onwards to ME3, I recommend Samara over Morinth for some small story elements.

Samara will also provide some war assets while Morinth will not show up at all (not even in the Citadel DLC) and provide nothing outside of that Banshee fight.