Luna was the only girl who saw Harry for himself, and accepted him completely. Luna was the one that Harry talked to and confided in, even when he wouldn't speak to anyone else. She was clearly the most perfect girl for him.

I like Ginny and Neville together too. I just really think that the last year at Hogwarts, trying to keep spirits up and survive with the Carrows would have bonded Ginny and Neville in a way that JKR didn't show. You don't go through those experiences together and not become close.

That being said, I don't think Neville would care anything about joining the Slug Club. Back to topic! :)

Samantha-Hope//Slytherin