SPOILERS. There, I said it. So don't read this if you are worried that it might ruin your experience with watching the movie, or reading the books, or both.
Most people fall into a few categories when it comes to adaptations. The first is "the book was better than the movie". The second is the exact opposite "The movie was better than the book". There's also a rare and lesser category "Both sucked", and an even rarer category "Both were amazing". Scott Pilgrim's adaptation fits into this ultra rare category, and I would go so far as to add on "Both were amazing in their own way".
Scott Pilgrim vs. The World (the movie) is a very condensed version of the series. Director Edgar Wright and writer Michael Bacall managed to find a way to distill the book into something concise, edgy, meaningful, and almost unclassifiable in terms of genre. Now, Scott Pilgrim vs. The World is two hours long, so one could guess that if they had made the film comic frame by comic frame, it would easily be four hours or more. Asides and subplots were cut, the importance of certain characters was diminished, and some were nixed all together, in favor of making the smoothest moving film that could be made about the most basic aspect of the Scott Pilgrim story - That Scott has to defeat the seven evil ex's of the girl of his dreams, Ramona Flowers, in order to date her.
In the film, we meet Scott as a care free twenty something who keeps his life easy and breezy, until he meets and instantly is entranced by Ramona. The League of Evil Ex's soon follows, and Scott, on seven occasions, has to fight these evil ex's. But what's really going on here? Scott is having to prove to Ramona that he isn't going to just cut and run, but we also find out that Ramona is having to do the same. As the ex's get more difficult to defeat, and as Scott gets more overwhelmed by them, it would be much easier for Ramona to cut and run, not have Scott resent her, and not have Scott get hurt by her and her baggage anymore.
The book begins in the exact same way that the film does, but as the book progresses, we get so much more. In the film, little bread crumbs are introduced that never go into the kind of detail that they do in the book, and, honestly, its a shame they don't. In the film, Envy Adams is introduced as one of Scott's ex's, a celebrity who's fame came shortly after they broke up, and who is still incredibly bitter towards Scott. In the books, Envy is fleshed out so much more. O'Malley takes a turn and revisits their past, something that started out with all good intentions, and ended, like most young love does, when Scott and Natalie (as she's known before she takes the stage name Envy) refuse to work anything out anymore. What we learn from the book, though, is that Envy still has a soft spot in her heart for Scott, that fights for supremacy against her white hot burning rage against him. While time hasn't healed all wounds for Envy, there is still a spark there, a memory of their past that would mean something if they could both get over their bitterness towards each other.
We also learn more about Scott's relationship with Kim, which, to me, is the biggest tragedy that the film left out. Scott and Kim were high school sweethearts that broke up when he moved to another city. Kim has never forgiven him for this, even though they play in a band together, and, we find out later, she still loves him, and that her deadpan and sarcastic (and sometimes hateful) attitude is her weapon against him, the way that she pays him back for breaking his heart (even though she desperately wants him back in her heart of hearts). We also find out that he left, basically, without even telling her, treating her as though the relationship they had was like a fine mist, having simply dissipated as they moved forward through it.
In fact, most of the differences between the movie and the book have to do with Scott facing the realities of his past, a past that he has erased and rewritten in his mind, to the detriment of his current life. While, in the book at least, it seems as though everyone is attacking him, we find out that this is for good reason. That he has lived with his head up his ass for so long that he's painted his own reality that is far removed from the authentic world. I feel like this is an interesting point, and one that is very lightly brushed over in the film. In the end sequence, when Scott has to fight Gideon, he sort of makes amends for a lot of things, but some of them seem kind of empty because we haven't really witnessed the back story of all of it. Kim, for instance. In the film, he simply apologizes to her. We are left to infer that their relationship was not the best, but it's not explained in the same way as the book (the fact that Scott cared so little about Kim that he actually told a mutual friend he was moving, and she heard it from this friend before she heard it from her boyfriend).
I think one of the most important things, though, that the movie and the books have kept intact is the parallel relationship between Scott and Ramona. Both are having to grow up in order to save the relationship they've grown to love. Scott is having to face all of the damage he's done to the women he's hurt in his life so that he can gain the self respect he'll need to be the man that Ramona needs. Ramona, meanwhile, has to face the damage she's done to her "evil" exe's in order to free herself from the anger and jealousy that follows her around like a dense shadow, in the form of the League that ruins every relationship she has, and only grows in size as she breaks more hearts. She has to learn not to run from her problems, but, instead, face them and work them out.
There are a lot of little things that you can spend paragraphs and pages talking about, but if you've watched the movie, and not read the book (or vice versa), then its better you discover them yourself. Ultimately, what it comes down to, is that both are amazing in their own way. As far as overall character development goes, which would be my favorite? I'd have to lean towards the books. There's just so much more that makes them satisfying story-wise. But the movie… The movie is an amazing cinematic experience that should not be missed.