You can find the answer to this at the end of Chapter 8. It is about a page or so from the end of the chapter -- page 128 to 129 in my copy of the book.
What Cherry says is that she just thought Bob was something special. He was not just like everyone else -- in fact he might have been a bit better than everyone else. Whatever it was, he was different and people noticed him. That is what attracted her to Bob.
Pony thinks that the same thing was going on with Dally. She thought of him the same way and that is why she thought she might fall for him.
Bob Sheldon and Dallas Winston have one common trait that attracts Cherry Valance to them: They are both dangerous. Although Bob is described as "sweet and friendly," he becomes violent after a few drinks. Although Cherry claims to hate this side of him, she nevertheless stays with Bob. Dallas's charm is obviously laced with a dark side, and this is precisely what prompts Cherry to admit to Pony that "I could fall in love with Dallas Winston... I hope I never see him again or I will." Cherry may appreciate Ponyboy's intellect and his introspective side, but she prefers a bit of wildness in a boyfriend.
Cherry Valance is attracted to both Bob and Dally because they are dangerous or typify the “bad-boy” personality.
"That's okay," I said sharply. It wasn't Johnny's fault Bob was a booze-hound and Cherry went for boys who were bound for trouble.
Dally’s reputation precedes him, and Cherry knows him even before the spectacle at the movies. She is attracted to him because of what she has heard, which is further compounded by his behavior at the movies. Cherry is probably aware of what Dally has been through, and when they first meet, she sees a strong individual capable of protecting her. Dally is also very confident, and despite being a greaser, he approaches the girls and talks to them regardless of social class boundaries. He also gets them drinks, showing he is not incapable of being a gentleman. Not much is said about Bob, but his predisposition to violence makes him share similar traits with Dally. In addition, both Cherry and Randy attest to Bob’s good nature. Thus, the two characters demonstrate their strength and frustration through violence, but they also have some good in them.