In terms of the crowd they spend time with, the main difference between Jake and Robert Cohn is that Jake belongs.
Robert is always an outsider, because of his personality, his choices, and the fact that he is Jewish. Robert was a boxing champion at college, and had success with his novel. These appear to be flukes, because every other aspect of his life is mediocre. He lets the success of his novel go to his head, and leaves his wife. He believes he is important, yet usually allows himself to be the butt of jokes, and is pathetic in his attempts to belong.
Jake carries his insecurities inside. He is well-liked by all, and possibly loved by Brett, who is as damaged as he is. He is aware of the subtle nuances of the company around him, where Cohn is oblivious. Jake understands the price of things, both monetarily and emotionally.