I read both series, and enjoyed both of them. But, my vote definitely goes to Harry Potter. There is no contest! Harry Potter held my interest from middle school (I started reading just before the 3rd book came out) through college when the final book came out; there is no way I would have stuck with Twilight that long. Like those posted above, I too am sad that Harry Potter is done.
Now I prefer neither, but when I was younger I always watched Harry Potter. So Harry potter ftw. Also I don't really like romantic-like vampire movies...
Never read Twilight, so I will have to Harry Potter. But there are books better than Potter that is less popular for no reason. The Bartimaeus Trilogy is rather good, but somehow it seems that noone knows it.