Beowulf has a lot more white on his face and body. He also has bigger eyes and more of a lanky build. And he also is the one wearing the black collar. We've often suspected that Beowulf is the younger brother (although they were both born in the same litter), because he has a more youthful, wide-eyed look and expression.
Arthur is mostly orange with white paws and a white chin and belly. Arthur has more white in his eyes and when he looks at you, he has this wise expression which makes him sometimes look like an old man. He's also the one wearing the blue collar.