Poe is a master of creating first person narratives where we suspect that the narrator is profoundly unreliable, and these two excellent stories are no exception. The biggest similarity that can be identified between the narrators of these two stories is the way that they are clearly shown to be insane to a certain degree. Note how the narrator in "The Black Cat" finds the second cat so abhorrent and what kind of emotions a simple cat stirs within him:
I am almost ashamed to own--yes, even in this felon's cell, I am almost ashamed to own--that the terror and horror with which the animal inspired me, has been heightened by one of the merest chimaeras it would be possible to conceive.
He is a character who is struck by unreasonable terrors and whose actions are based on these fears.
Likewise, Montresor is shown to be a bloodthirsty narrator fixated on achieving the most terrible of revenges:
I must not only punish, but punish with impunity.
The way in which Montresor punishes Fortunato so terribly for some supposed slight whose nature is never explained seems to suggest the insanity of the narrator.
Considering differences, it is clear that Montresor has carefully thought through his crime and how to commit it without being caught. It is definitely a premeditated murder, cunningly carried out so that Montresor remains free. In contrast the narrator of "The Black Cat" kills his wife in a fit of anger, and then quickly has to decide how to conceal the body.