Vito, you asked a good question, and SB and Carstonio gave you excellent and succinct answers. I'd just add my two cents:
The novel provides more rounded development for some characters. It also has some terrific stories that weren't included in the film, such as Vito's rise, Sonny's rise, Neri's introduction to the family, and how the Bocchicchios helped bring Michael back from Sicily. On the other hand, the novel has a generally rough-edged, hurriedly written quality to it, while the film is nothing if not polished.

Like Carstonio, I found the roman-a-clef aspects irritating. Puzo was one of those authors (like Ian Fleming of James Bond fame) who couldn't resist putting into his novel every bit of erudition he ever accumulated--whether or not the erudition had anything to do with the plot. Johnny Fontaine serves a highly useful purpose at the beginning by helping to define Vito's influence and the range of his caring--excellent. Thereafter, Johnny, and Nino, serve only to enable Puzo to show off what he learned about Hollywood. That Hollywood showboating BS ruined "The Last Don," IMO. And that whole bit about Lucy and Jules was designed so that Puzo could show off what he knew about her operation. Evidently a female relative or friend of his had had the operation, told him about it--and presto, it gets into the novel. It's not only boring, but after a while, you feel as though Puzo took advantage of your interest in his novel.