The poster is meant to be a teaser poster, meaning it's purpose is to draw in the audience and reveal just one teeny bit. Physiologically, the human instinct is to glance at a picture and grab onto the main focus. For Breaking Dawn, there is nothing to focus on apart from the words! It's just a sunset background, a forest and a couple of words. Obviously, Twilight's fan-girls swallowed it down as if it was the sweetest thing ever. But how do you explain the fact that the teaser poster is not teasing for the eye at all? Even my art teacher could do better than this! It's almost like the movie art designers have given up, "This whole movie rots anyway, let's just stick this on and get the whole thing over with", joking, but still.
The Harry Potter poster is still plain. But SMACK! Hogwarts. In ruins. Hogwarts. On fire. A war. How is it possible that Hogwarts didn't withstand Voldemort's troops? The poster makes you think; it tempts you into the cinema. It teases you.

Um, they are both equally effective in letting me know the respective movies exist.

I love the Deathly Hallows teaser poster. The image of Hogwarts burning gets me every single time, it's as if my childhood is just crumbling. We've grown up looking at this magical place and all of a sudden it's being destroyed. It's eerie. It's quite terrifying actually.
I love the Breaking Dawn Part 1 poster also, it's simplicity is nice, it's unusual and different. But the HP poster is brilliant.

yup

The HP one draws you in more. If you've already read the books, or seen the previous movies then the iconic image of Hogwarts burning let's you know straight away what it's about and the prominent HP7 at the front with the date on it lets you immediately know when the movie will be shown. If you don't know anything about the series, then it's more likely to catch your attention and make you want to find out about it. The Breaking Down one kind of looks like a poster for a nature documentary.