In having a conversation the other day about Faith vs. Buffy (focusing primarily on season three since I am trying to keep up with GAKS over at potentialcast.com) I had the following thoughts:
(1) Faith is set up, not necessarily as the dark Slayer, but more as the lower-class Slayer (vs. Buffy who is obviously middle class). But what is odd is that the clothing, which is the key feature in determining difference between the two girls, are actually VERY similar to Buffy's own clothes in S1. All that bad leather and dark colors, it's like we're getting a repeat of Buffy rather than the reverse of her.
Faith's class status lead me to think about:
(2) Family/Community as a human need... Buffy's constant ability to avoid death is because of her ties to the Scoobies. She is the longest lasting Slayer because of her emphasis on community. In all Whedon's works, community and family are necessary to human survival. Faith doesn't have this basic human need. And what we see of her in S3 (along with what we will later see in Spike, to a degree, and even possibly Andrew and Jonathan) is that humans will go (in this 'verse) to great lengths to find and keep any semblance of family. The Mayor is Faith's downfall because he gives her exactly what she deserves: a sense of community. If she has to lie, steal, cheat, and kill in order to keep the feeling of safety that the Mayor provides, she will. It's not a matter of choosing evil over good, but rather the safety of a relationship over the fear of being alone.
And my final point:
(3) The main difference between B and Faith is what makes them so similar: their complete dependence on perception and performance. Faith wants people to think she is selfish and Buffy wants people to think she is selfless. That's it, people. That's what divides them. Neither is more selfish or selfless than the other. It's that Buffy is selfless in front of others... she makes big speeches and big appearances and makes big actions that others can see. But B is selfish in the dark, where no one can see. Faith is the opposite.
I find this all fascinating, to say the least.