Both. For a relationship to be successful in the long term, you have to be attracted to your partner both physically and mentally. If it's on looks alone, it won't be long before you get annoyed with her personality and want out (unless you just happen to hook up with a hot girl you know nothing about and it turns out she has a personality that you love). If it's on personality alone, you might last longer but eventually you'll lose interest, take interest in other, more attractive girls, and (I predict) think of her more as a good friend or sibling than a partner. And, as others have said, looks are what draw you in to a person, because 99% of the time we see someone long before we can get a sample of their personality. At the very least you would probably feel there was something missing in the relationship. Unfortunately, it also works the other way around, and that's probably why my last girlfriend broke up with me.