Red Dragon is much better in book form and a lot of the holes get closed up in it. If I remember correctly a good bit happens inside the characters' heads and it's really hard to convey that in a movie. If you have time I *strongly* suggest reading the book. It's worth the time.

Hannibal, on the other hand, is probably better on film-- I remember the book being really draggy. I was NOT happy with the way the book ended but I've never seen the movie because I didn't want to see a particular scene.