In terms of goals and dreams, the two differ in significant ways. Tom Wingfield is a dreamer, longing for a life of freedom and adventure. He longs for a life that stands in direct opposition to the miserable life he leads in St. Louis, supporting his mother and sister, working each day at a warehouse job he detests. Tom has dreams, but they are vague; he formulates no specific goals to achieve them. He simply wants out. When he finally breaks from his family, Tom just runs away with no particular place to go.
Jim O'Connor also has dreams, but his are expressed in terms of specific goals. Jim wants to succeed in a career; he wants economic success. He believes in the future of broadcasting, the rise of television. Jim prepares for his future, or tries to, by taking night classes in public speaking. By working to achieve his dreams of material success, Jim pursues what is generally considered to be the American Dream.
Tom is also stifled in his attempts to reach his dreams by the fact that he feels responsible for both Laura and his mother. Jum has more freedom to pursue his dreams. Jim's dreams are also more "socially acceptable" whereas for Tom, a gay man with a desire to be a writer and with a single mother and disabled sister to take care of, his dreams are much more out of reach. Tom is a highly autobiographical character. Tennessee Williams experienced much of what Tom experiences in his own life. His sister, Rose, on whom Laura is based, had a problem with mental illness and was ultimately given a lobotomy. Tennessee Williams lived with a degree of guilt over this for much of his adult life. Williams himself was a gay man who wanted to write, and he often had times when the dream was out of reach. A good editor who stood by him, going so far as to allow him to retreat to a country home and take care of chickens for a period of time in order to refocus, ultimately is what allowed him to break free. (all of this can be found in his letters that are collected in the play "A distant country called Youth") Tom is trying to break free as well.
Essentially, Jim's dreams reflect a socially acceptable norm; whereas Tom's do not. This is key to the difference between the two characters as well as to a degree of strife that exists between om and Jim themselves.