They are equally cunning, manipulative, determined and indifferent to human life, but they are not crazy. However, their motivation is different.
Montresor is motivated by revenge. He needs to exact revenge on Fortunato to preserve/rescue, his family's honor. He is acting on a family tradition that propels him to plan and execute a perfect murder. He lures the unsuspecting Fortunato to his death without violence, there is no evidence.
Zaroff is motivated by his desire to be entertained through the sport of hunting humans. He has grown bored with hunting animals, so he matches his wits with human prey. He gives them a chance to defend themselves, so it is not too easy for him. He thrives on the challenge of tracking a human, who is smart, with higher level thinking, rather than pursuing a simple animal.
Montresor is not a classic murderer, he has chosen his victim for a specific reason, a thousand injuries against him, Poe does not explain what they are, he is not likely to murder anyone else. So he is not crazy, just filled with purposeful revenge. His behavior is not normal, but it is easier to understand than Zaroff's.
Zaroff's behavior is more like that of a mad scientist who gets a thrill when he makes an unethical discovery in his lab. Every time Zaroff hunts a human he must get a rush of adrenalin, like riding on a roller coaster. It's easy to get addicted to this feeling.