I'd have to say Lindsay for me. She drinks, she smokes, she's a b!tch and she has fake boobs. I'm sorry, but that's just my type.
Hollywood seems to be pushing out these teen (or "tween") celebs like crazy now. They push 'em out, market the cr*p outta them and then drop them. Hilary is making movies, music and has a clothing line. Lindsay is the same, minus the clothing line which i'm sure is not far away (unless she already has one i'm not aware of). Hollywood just finds good looking girls, gives them songs to sing and puts them in some cr*ppy movie and make them famous to make money. Add in a bit of controversy(like one stealing the other's bf) and you got big bucks. I dunno, but thats' what I think.
Anyways so back to the point, if I had a choice I'd pork lindsay first, then hilary. then both. then lindsay again.
But in all honesty the hottest young celeb is Kristin Kreuk. She's just hawt. I'd marry her in an instant.