For one thing, Nick comes from a relatively affluent family and received a college education from an ivy league school; Gatsby, on the other hand, comes from poorer folk and only went to Oxford for a semester after the war as part of a program for soldiers. In the first chapter, Nick tells us that he "graduated from New Haven in 1915"; he went to Yale University, as did Tom. He also says that his "family have been prominent, well-to-do people" in the Midwest for several generations. Nick hasn't had to work and scramble the way Gatsby has. Gatsby also seems to possess a drive that Nick does not necessarily have. It's not that Nick is lazy, but he isn't driven and restless the way Gatsby is. Although he says that he came back "restless" from the war, he only needed to move to New York to "learn the bond business" in order to satisfy himself. Gatsby, on the other hand, had much grander dreams: he wanted to amass a fortune, turn back the clock, and recapture lost love with his former lover, Daisy. He had, as Nick says, "an extraordinary gift for hope" that seemed to separate Gatsby from just about everyone else.

I think when Nick says this in the introduction it explains their differences, and the way that Nick sees Gatsby.

“If personality is an unbroken series of successful gestures, then there was something gorgeous about him, some heightened sensitivity to the promises of life,…[Gatsby had] …an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again.”

THere is also this exchange which highlights their differences:
“You can’t repeat the past.”

“Can’t repeat the past?” he cried incredulously. “Why of course you can!”