Let's start with the similarities.
First, General Zaroff and Rainsford are hunters. They both love the hunt, and both men are accomplished veterans. We can say that hunting runs in their veins. We see this even in the beginning of the story when Rainsford says to his friend, Whitney, that there are only two classes in the world - the hunted and the huntee.
The world is made up of two classes--the hunters and the huntees. Luckily, you and I are hunters. Do you think we've passed that island yet?"
Second, both Rainsford and the general are cultured men. Even the ability to hunt presupposes a certain amount of wealth. Moreover, Rainsford, in his interaction with the general, can appreciate his worldly sophistication. In other words, it takes a sophisticated man to appreciate it in another.
Now for the differences.
Zaroff takes hunting to a new level, which is unimaginable to Rainsford. Zaroff even boast of creating a new animal by which he means humans. When Rainsford finds out about this, he is disgusted and wants to leave the island immediately. From this we can say that Zaroff is mad (as in crazy). He takes his love for hunting to a perverted degree. Rainsford does not. From this perspective, they are different men.
Here is a dialogue that shows the difference between the two men:
"I wanted the ideal animal to hunt," explained the general. "So I said, `What are the attributes of an ideal quarry?' And the answer was, of course, `It must have courage, cunning, and, above all, it must be able to reason."
"But no animal can reason," objected Rainsford."
"My dear fellow," said the general, "there is one that can."
"But you can't mean--" gasped Rainsford."
"And why not?"
"I can't believe you are serious, General Zaroff. This is a grisly joke."