I voted for Faith, largely because she is still alive. I do wonder how Faith would deal with hypnosis or Drusilla. I suspect, given how Faith doesn't mind taking the unexpected and dirty route to get a job done, that she would have been fine.
Like many here, I think the fact that Kendra was killed off so early makes comparison difficult. Bearing that in mind, I'd like to make a small caveat to my choice. Early on, Faith did seem a bit sloppy and cowardly. If I'm basing both slayers on their first appearance, in a head to head battle, Kendra probably would have wiped the floor with Faith. However, Faith eventually got over her fears, and became as self-sacrificing as Buffy to accomplish her means. In some ways she's even more willing to sacrifice herself than Buffy. The new Faith seems to me to look at her life as expendable, and is more than happy to take the first choice that will bring her foe down with her.