Jay Gatsby and Tom Buchanan are both rich, but they are very different people.
Each of them is willing to put his personal interests ahead of the interests of others. We see this in Gatsby when he insists that Daisy denounce her love for Tom. Gatsby is willing to ask Daisy to say something that is very difficult for her to say in order to gratify his own desire.
Tom carries on an affair and hits his girlfriend, showing that he is quite willing to disregard the well-being of those closest to him (his wife and his girlfriend).
Tom's money is "old money" and he is perfectly secure in his wealth. Gatsby has raised himself out of a state of relative poverty and so represents "new money".
Tom is not very bright and is impressionable, as evidenced by his speech at the dinner early in the novel where he talks about a racist book he has been reading.
He tells Nick that, based on a book Tom has read and obviously reveres, “The Rise of the Colored Empires,” civilization is “going to pieces”...
Gatsby is not impressionable at all, but is instead rather extremely single-minded. Where Tom is jaded, cruel and bitter, Gatsby is hopeful and dedicated to the pursuit of a fantastic (and somewhat innocent) dream.
Gatsby believed in the green light, the orgastic future that year by year recedes before us.… Tomorrow we will run faster, stretch out our arms farther.…
Gatsby builds up his great wealth for a purpose - to attain Daisy's hand in marriage - and all of the power he accumulates is directed toward this end. He is an idealist and a dreamer.
For Tom, status and power exist as an accepted way of life. It is his due, as old money, to wield power and authority. There is, for him, no goal, or ideal, or aim that justifies or organizes his power and his personality. We see Tom in the end as a bigot and a coward.