I personally don't know why people are comparing the two to each other.
Harry Potter is about witches and wizards who go a magic school and about how Voldemort is pretty much trying to murder Harry. Not saying anything is wrong with Harry Potter, i loved the series.
Then we have the Twilight Saga. So different to Harry Potter! It is about a girl and a vampire who fall in love, and have to try keep the girl safe from other vampires wanting to kill her, and turn her into a vampire. (Basic Outline of story)
Now, if I was going to chose, I would say The Twilight Saga, purely because the book just totally draws me in, and I am suited to that kind of book. Not saying that Harry Potter wasn't great, I just prefer Twilight.
No Twilight hasn't sold as many copies, but to be honest, I wouldn't be giving a 10 year old Twilight to read. Harry Potter appeals to a wider range of people, including adults, however, Twilight appeals to a less wide range of people, mainly consisting of pre-teen and teenage girls.