A few observations...

The film is definitely better. The book is not as well written as I remember it, and altho it was a huge bestseller, I don't think anyone will ever confuse it with one of all-time great works of fiction.

In just about every case where dialogue in the film was lifted directly from the book, the film version changes a word or two for the better. Some examples:

Sollozzo (book): "...blood costs too much money"
Sollozzo (film): "...blood is a big expense"

Hagen (book): "Luca Brasi is sleeping on the bottom of the ocean"
Tessio (?) (film): "Luca Brasi sleeps with the fishes"

Don Corleone (book): "See how they have massacred my son"
Don Corleone (film): "Look how they massacred my boy"

Don Corleone (book): "Tattaglia is a pimp. He could never have outfought Santino. That's why I don't have to know about what happened. It's enough to know that Barzini had a hand in it."
Don Corleone (film): Tattaglia is a pimp. He never could have outfought Santino. But I didn't know until this day... that it was Barzini all along."

Puzo does, however have a rather unique style of writing, IMO. Kind of like if the book was originally written in Italian, and then translated to English by someone who wasn't completely familiar with all of our colloquialisms. Here's one example: talking about
"Racing tracks" (instead of racetracks).

Everyone should absolutely read the book. It's a necessary companion piece to the film because it fills in a lot of character development and back story. But I think it's safe to say that its the films which have brought us all here, and for good reason. There'd be no website like this if there was only a novel