In Cherry's eyes, Bob and Dallas share destructive qualities.
Cherry sees similarities in both guys. She is dating Bob, but could also see herself being with Dallas: "I could fall in love with Dallas Winston... I hope I never see him again or I will." As a result, Cherry is in a unique position to judge both characters. While Cherry appears to "have it together," she has a tendency to be attracted to tough and provocative guys. Both Bob and Dallas share this intense, "hard to handle" quality.
One similarity that Cherry sees in both is their toughness. Bob is the leader of the Socs. He is violent and very tough. When he has a couple of drinks in him, his toughness manifests into violence. In a similar way, Dallas is the tough leader of the Greasers. He displays his authority in the same way. Violence is a means of conveying toughness. Both Bob and Dallas represent a quality of toughness communicated through violence. These destructive qualities make both boys distinctive to Cherry.