I prefer Harry Potter, to be honest. The Twilight Saga has potential as a story, but I was soooo not impressed with Stephanie Meyer's writing. On the other hand, J.K. Rowling is a masterful writer who draws you into the story and adds dimension to almost every single character she creates.
The hallmark of a good writer is to start with an imperfect character and then grow them and mold them throughout the story until they are something better (or worse?) than what they were to begin with. I can honestly say that Stephanie Meyer's characters stay rather stagnant (except Jacob, who evolves almost constantly throughout the books) until the last book in the series. It's like she got to the last book and realized that her characters were pretty much exactly the same at the start of the fourth book as they were at the start of the first book and that people were starting to notice, so she hurriedly added some explosive growth to them and hoped for the best. I think that's why most people consider the fourth book to be the best of the series, because there was dimension in that book that was lacking in the other books.
With the Harry Potter books, there were constantly scenarios that changed and shaped the characters, bringing new traits (both desirable and undesirable) to the surface. The characters weren't always perfect and self-sacrificing. They were what humans should be, with human failings and short-comings, along with courage and nobility, however deeply buried. Even the minor characters in the Harry Potter books had room to grow and used it to its full advantage.
Now that I'm done sounding like the English/education major I am...lol.
Honestly, if one of my students were to do a book report on any book in the Twilight saga (with the exception, possibly, of Breaking Dawn), I would probably give them a non-passing grade and make them redo it...or just veto the book from the get-go. This doesn't mean that I haven't read all four of them myself...indeed, I have, but could only bring myself to do so once. On the other hand, I am on my third time through the Harry Potter series in the past few years (I'm also a Robert Jordan fan and since there are 14 books in the Wheel of Time series, I have staggered the two series in my reading...interspersed with a few of the 50-ish books written by Clive Cussler, another of my favorite authors).