An interesting question. The first distinction is in intention. Language in fiction can and often is intended to be pleasant in itself. Writers pay attention to sound, cadence, the literal shapes of words, etc. They might form a line for how it sounds, or for its beauty. Scientists can write beautifully, but their intention is to communicate and persuade.
A second difference is related: the relative weight of connotation and denotation. Language in fiction is always charged with connotative meaning, and is selected for emotional impact; language in science tries to be denotative.
A third difference is in focus. Fiction is about people, or about characters (animals, robots) with characteristics related to humans. Science is about the world, and can leave humans out entirely, or, if they are included, can treat them as objects rather than subjects. This means fiction uses more dialogue, more individuated descriptions, more active verbs, etc., than science.