Buddy is the narrator and central character of both of these stories. We see the world through his eyes, and are thoroughly on his side (and against the other guests) in "Roof Beam." The storyline follows Buddy, not Seymour, and the central conflict is built around his character. In "Seymour," you could argue that the reader is still closer to Buddy than to Seymour.
On the other hand, you could also argue that, while Buddy is narrating these tales, they are fundamentally about Seymour. "Roof Beam" is about Seymour's wedding day. "Seymour" is about Seymour and his life in general. He's even the title character.