I think the problem is that's entirely subjective. What I read of Potter (maybe a third of the first book?) I found poorly written and imagined -- which is to say that's more to do with what kind of reader you are, what you want from a book, than any essential quality of the book itself.

Clearly enough people found both a rich fantasy world that fit their reading needs -- they both sold huge amounts of copies. And anecdotally, we had several very broad and intelligent teenage readers who were store regulars who utterly flipped out over Twilight -- clearly it met a need for them.

What I'm hoping to get at is why the general discourse seems to feel as you said: that Twilight is some kind of evil.