Well, as mentioned, it depends.
If you read the book, then you will have certain knowledge that people who did not read the book, does not have. In some cases this might help you understand the plot of the film a lot better, and give a second layer of insight, which then can be used to enjoy the movie even more, in others, you will find yourself annoyed that some of your favourite parts of the book might not be included, are played out wrong or doesn't add up to what you expected from the book.
Reading the book first also helps you understand the setting better in most cases, but will ruin secret plot endings to you, which might ruin the film to you as well.
On the other hand, watching the movie first is often a great kickstarter to get into the book's universe, and might spark an interest in the series, the author or the genre. It is also easier to fill out the blanks in between the plot shown on screen, and what the author describes in their story. For some people it is also easier to start with a set standard for their imagination, instead of building up the story mentally themselves.
However, for people with less interest in reading (and, if the book is particularly badly written, regular readers), it might become an annoyance that the plot often moved quite differently in a book than it does in a film. A film might be interesting to viewers because it is filled with action, but the book might be somewhat philosophical, and uses a lot of time to get from one point of action to another, which could annoy some readers. Or it could be littered with bad writing that does not appeal to the regular reader, while the movie at least makes it slightly more entertaining, or at least gets it done and over with quicker.
So, it is pretty different, it depends on who you are and how you like your stories told.
However, if you are going to read the book, then watch the film, it is better to see the film as an individual storytelling based on the same event, rather than a book turned to movie, because otherwise there will be disappointments.
Likewise, when reading after you have seen the movie, keep in mind that this is the same story told, well, by someone else.
After all, films have 2 hours to tell you the plot of an entire book, while also showing you how the characters are, what they feel and why they are driven forward.
They don't have much time, but have the benefit of actions being shown instead of described. They define the character through their actions.
Books, on the other hand, has to describe actions in a believable and understandable way, and important events have to stand out in a way that is at least subconsciously remembered, so that references later aren't lost to the reader.
They then have the time to explore the character through their mind, their emotions, feelings, and can analyse and comment on the actions the characters take, and subtly apply pressure to the plot through various storytelling means.
If that makes sense.
I am the reader type, though I don't have much motivation or time to actually do that much any more. I do enjoy a good movie too, it is a good way to get your mind off of things.
I am more likely to watch a movie because I have read the book, than I am reading a book because I watched the movie, perhaps because I dislike being told the entire plot like that, or perhaps my imagination works fine on its own.
You try tell your friends that.
