I was thinking about difference in Harry Potter and Twilight background and this is what I concluded. Enjoy!

Knowledge of magical beings not soo very far from their backyards
Harry Potter: As I can get from HP books and The Tales of Beedle the Bard, wizarding world was known to Muggles, but because of witch hunt and victims of it in both sides, wizard decided to go underground and not to be commonly know to people. They don`t use spells in front of Muggles and if sombody does manage to discover their secret, they erase their memory.
However, there are exeptions. Families of Mugle-borns ofcourse know what their children are and Prime Minister(s?) is informed of latest events going on in the Wizarding world because of Ministry`s protocol, that they must notify Muggles. Probably happening all over the world and as it has been said in the Half-Blood Prince, who would belive the anounce of existence of magic anyway. The chance of individuals that were clever enough to found out about the wizards and and stay quiet knowing what would happen to them, if they show what they know is never mentioned but not opposed either.
Twilight: Carlisle explains, that vampires were haunted and that he used to be a vampire slayer himself, but, corect me, if I don`t know something, never how come they are not haunted anymore. They just seem to...disapear to people.
They have the Volturi, which are obviously woried about people finding out about them (something tells me, that average human being wouldn`t be as happy about it as Bella). However, the only thing they can do is killing the avare human (or turn him/her into a vampire). And this rases some questions. Has nobody ever menage to tell vampire-thingy forward to a great mass of people? Haven`t any videocamera recored a vampire doing things normal human could`t (eg. stoping cars and throwing each other around)? Has`t any vampire ever been so disgusted over himself/herself to warn people? Vampires don`t just live next to humans, they also eat them. I understood, that there is at least a few hundred vampieres. OK, I will say about 500 (probably more) that are not nice vegeterians, which seem to be minority. Lets say they are on a diet (probably not) and they eat just once on 4 weeks (probably more often). Year has 52 weeks so 52/4 x 500, which makes at least 6500 weird corpses per year. Yes, some can be hidden, but honestly, this is a lot of dead bodies. In Mexico, a few (of a disease) dead cows made the Chupacabra mith, and Meyer wants to say that 6500 dead people (per year! and don`t forget, even "vegeterian" vampires eat animals) without blood wouldn`t couse a mass panic?! And besides, there is diffence between not telling around your neighbor can make a table float with nothing but stick in his hand and not telling around that he eats a person or two per month.
I leave the chance of vampires who can erase memories, but still, there is a difference in few thousand people who forgot and few thousand people who died.

Blending in
Harry Potter: Wizards, witches and magical beings live all over the world and have grat problems staying unnoticed. But wizards are only humans with magic powers and they also look as such. They often don`t know how to drees like Muggles, but hey, weirdos are still humans, right?
They often live side by side with Muggles and try to assimilate.
Twilight: Vampires mostly travel over the land in smal packs eating people, which is not realy sicretive (not James Bond aproved, also anti-spyish). There is documentary for every Puma attack, but bloodles bodies obviously go unnoticed. Other vampires like Cullens and Volturi live in big, stacionary grups, eating bears/tourists. Again, hard to hide. Plus the fact that they sparkle doing it.