Okay. I'm going TRY to be thorough, and express my reasons in a clear way. If there's something you think is wrong or incorrect, feel free to debate that point.
The plot of the book is like the structure of a building-without it, the book is moot. Both HP and T have simple plots (Girl falls in amor with a vampire, boy discovers he's a wizard).
However, HP (In my opinion) is unpredictable, whereas Twilight is very predictable. In fact, (I swear this is true) I had a person who'd never read the Twilight books before, watched the movies, and had barely known the series existed, read book...