2 girls and Sadies. What should I do?

Ok so theres two girls. The first girl asked me to sadies today. The second girl didn't actually ask me yet, but she asked me if i was going to go to sadies a couple days ago, so I think she might ask me pretty soon. The first girl is cool and stuff but to be honest, i'd rather go with the second girl because I like her more. It might sound messed up but I'm just telling the truth and I don't know what to do. When the first girl asked me today, i just said yeah because I didnt want to say no because she asked me in front of her friends and they said she was really nervous about asking me and if i said no I didn't want to make her sad. And i couldn't tell her "oh well im waiting for someone else to ask me" either. But i really want to go with the second girl but she hasn't asked me yet. In case she does ask me, what should I do? Go with the second girl and say "sorry i told you i would go with you but im gonna go with this other girl" to the first one? Or how should I put it? Because I REALLY want to go with the second girl. Help?