There's no real good answer to that question. In most cases, read the comic/manga before, so you get the whole story.
But in the case of SP, the last volume was published on the same day the movie was released. Aesthetically and tonaly speaking, they are different. If you watch the movie before, it will give you a fucked up sense of Scott's personality. Comic Scott is a lot more fun than Movie Scott.
Read Volumes 1-3 in a single go(they where published fairly regularly), then read volumes 4 and 5 (there was a wait in publication where fans eagerly anticipated the next books), watch the movie and then read volume 6. This is the way that most people experienced the whole work, and I think it's reflected in the development of the comic.