From Buffy's point of view I think that the reason why she was initially originally willing to be a part of Cordelia's group despite Cordelia obviously being a bitch was because they had represented familiarity to her. I just finished reading "The Origin" comic, so this post is coming off of the heels of that. After the events of "The Origin" Buffy's state of mind in the pilot episode makes perfect sense. She's in a new place, fresh start, clean slate, and she wants to put all of that Slayer nonsense behind her, move on, and go back to being a normal girl. Cordelia represents the girl that Buffy used to be before the Slayer destiny was burdened upon her, not only that but Cordelia & the Cordettes in general gave her a familiar social element that she was used to in this brand new environment. The thing is though Buffy realized (midway in "The Origin" story I think?) that her popular friends weren't actually real true friends who had cared about her (I mean, they really didn't care her at all given their comments at the end the comic!). So even though, in her mind, she may have flirted with the idea of becoming a Cordette because she was used to naturally always being a part of the popular girls social group, once she had saw how Cordelia had treated Willow it probably reminded Buffy of how her old friends were towards her once her life changed with the Slayer destiny back in LA.
Reading "The Origin" gave me a much greater appreciation for Buffy's friendship with Willow than I already had before because Willow was Buffy's first real friend who had truly cared about her. They probably would not have been friends at all had Willow met Buffy a year or two earlier. Who knows? If they had met earlier maybe perhaps Buffy would've said a similar Sears comment to Willow and lost out on one of the greatest friendships of her life. Willow and Xander truly were the best things to come out of the Slayer destiny for Buffy. They really cared about her and her well being in comparison to her old "friends".