Buffy was very obviously the one true Slayer. Even if Faith was the Slayer too and all of that. It was the same when Kendra was alive, even though 'offically' a new Slayer had been called, there was no denying the Buffy was the one true Slayer. I think people can sense that, it's obvious in the way she carries herself also.
I remember in one episode in Season Seven Buffy was listing who was going to get killed of. It was the Potentials, then Faith, then her. I think she's right. Because Buffy is more of a threat to begin with but she is the that everybody wants and needs to kill and so they'll make a big deal out of it. Faith can't really be connected to the Slayer-ness as much anymore due to her turning away from it and then all that followed. So it was Buffy they would kill last. It wasn't a matter of logic really.