Faith was, is and will always be anti-Buffy. She was the darker Buffy, she is redempted but is not "light" as Buffy and will not be in the Buffy's league. Slayers like Buffy are more responsible and have more awareness about being a slayer (like Kendra), slayers like Faith are more skeptical about the role of the slayer (like Kennedy). In short, Buffy and Faith are in different leagues, call it different houses [reference to Harry Potter].
I completely agree, they are different, different personalities, majorly different pasts and present lives and they have different methods and moralities. So theres no real grounds for comparison other than to say they are different, i mean you could possibly be considered ethnocentric for even suggesting one was better than the other