thepunktheory
First the novel. C. S. Lewis spent a lot of time describing who the kids felt back in Narnia and what happened on their way to Caspian. In fact the whole back story and preparations make up more than 2/3 of the book. The battle itself is actually only a rather small part.
So due to this you get very detailed info but at some point it becomes annoying and the story seems to drag on before it finally can pick up speed again when the battle begins. All in all the novel is a decent read although it didn't excite me as much as the first part.
However, the characters are very likable and it's easy to follow the story and care for them. Although the story may become a bit boring at some point, you really want to know what happens next. Almost impossible to put the book down!
Let's move on the film. Here the priorities shifted. While the battle only made up a small part of the novel, the movie almost solely consist of fights. They even added in another huge battle. I actually was grateful that big chunks of the Pevensies poking around in the woods were left out as those were really not necessary to follow the story at all. Having so many fighting scenes was a bit irritating at first but it makes for a very exciting movie.
However, a point I wasn't pleased with is Ben Barnes as Prince Caspian. This character was depicted very different from what it was supposed to be in the novel. For me the Prince Caspian on the paper was not only more likeable but also felt smarter despite his younger age compared to the movie. His decision felt more thought through, while the movie characters seems a bit impulsive and intemperate. That really gave me a bitter aftertaste.
Furthermore, a few things in the film were confusing. Due to the lack of explaining scenes (that were provided in the book) a bunch of things, especially concerning Aslan's return, seemed weird as there was no context.
So summing up I have to say that Prince Caspian is a decent adaptation, although it is by far not as good as the first installment!