Buffy actually has friends and family that support her. Faith does not. Buffy isn't necessarily a better fighter or a stronger one, but she does have more inspiration to win a fight than Faith. Faith admitted it herself. Buffy has the friends and boyfriend and family. I believe that is why Buffy succeeds better in a fight than Faith. If it was switched the other way around Faith would win more fights and Buffy wouldn't. Buffy has been training so she has a better tecnique than faith plus all the time she was in a coma