I'll admit I never actually read Beowulf the 8th Century (or whenever) text. I haven't read The Iliad, the Odyssey, or Canterbury Tales either. But I did read Eaters of the Dead, which is based on Beowulf. And since I had another idea for E, I figured I might as well do this. If it makes you feel better I did watch the Robert Zemeckis movie from 2005 or so with that creepy Polar Express animation. It was OK but really that animation is weird. It makes it hard to lose yourself in the story.
Eaters of the Dead is a more realistic version of the story. It starts when an Arab poet is caught banging a sultan's daughter or some such thing and gets banished on a crummy expedition. He meets up with some "Northmen," who take him far north to a village that is beset by what people seem to think is a fire monster, but is actually a bunch of little Neanderthal-type dudes with torches and wearing bearskins. As the title suggests, they eat their dead, which is why no bodies were recovered until the Arab and his Northmen buddies find a way to stop them.
The movie version is called the 13th Warrior because I guess the other title sounded like a zombie flick. And also because the Arab is the 13th guy selected to go on the expedition up north. It's kind of a Fellowship of the Ring thing going on there. A lot of the movie is the same but there were two differences involving the Arab.
First, in the book the Arab never learns the Northman language. Which in a movie would be really annoying because you'd have to do everything in subtitles. So in the movie they show him watching the Northmen and picking up their language until he's more or less fluent. Realistic? No, but it's a good practical idea to make it easy on the audience.
Second, in the book the Arab is carrying around a huge Northman sword that he can never really wield and thus isn't all that important to the action. In the movie, he takes one of the Northman swords and turns it into a scimitar that he can actually use. Again, it's not as realistic but you paid for Antonio Banderas to star in the movie so you actually want him to be contributing to the fight scenes.
Basically the distinction is if you want more realism, read the book. If you want more entertainment, watch the movie. If you want to be creeped out, watch the Robert Zemeckis movie. If you want to be bored, read the 8th Century epic poem.