In what ways are Montresor and Fortunato alike and different in "The Cask of Amontillado"?
An interesting question. Montressor and Fortunato are alike in a number of ways. First and most simply—always start with the basics—they live in the same place and time. This allows them to engaged in odd behavior and excuse it by the idea that it is carnival time, and is essential for the plot. They share an interest in wine, and have highly developed senses of pride. They share secrets. They are both masons, though this is somewhat of a pun (one is a literal mason; one is a Mason, a member of a secret society).
They differ in that Montressor narrates the story (again, start with the basics) and so we see his side of things, but never Fortunato's. After that, they differ in the type and nature of deception. Montressor deceives Fortunato regarding his intent, actively lying to him; if we believe Montressor, Fortunato pretends not to know he's given offense. Finally, of course, Montressor is a killer.
Judging from the third paragraph of Poe's tale, both Montresor and Fortunato are specialists in art, "gemmary" (jewels), and other luxury goods, which would include expensive wines. This is how they know each other and how both of them know Luchesi, who is evidently not just a connoisseur of wine but someone who actually deals in it as an importer. These three men are evidently not members of the highest nobility who do not have to work to earn their livings. They are borderline upper-class traders. Venice at that date (the mid-nineteenth century) must have been full of aristocratic men forced to engage in some sort of trade to maintain their luxurious lifestyles, with palazzi and servants. If Fortunato had a title, Montresor would have addressed him by that title at least once. The same with Luchesi.