Well I never read the latter, but Go Ask Alice has some pretty adult themes. It's an extremely easy read though, I liked it a lot. I don't think either would be considered too young, they just happen to be about teenagers.
Harry Potter all the way. I read the Twilight and found it rather boring and I really didn't like the way it was written. I'm not a romance fan and I couldn't relate to any of the characters. Bella was unlikable and so was Edward, so I couldn't get into it. Harry Potter has more action and...