Evil Angelus 95
Total Dork
I think they both did fantastic jobs, I completely believed that they were each other's characters.
I've watched this episode almost to the point of memorization (it's my favorite episode of Buffy, so of course I've watched it a lot), and I'm always more impressed with Eliza. In the scene between her and Giles, she just nailed it -- I will literally watch that scene over and over and over (it's the inching comment). Though, I have to give Sarah a lot of credit -- she carried most of the episode, though I've never completely fell into the mindset of "this is Faith" while watching her, which has always thrown me off.
I will say that I'm slightly biased towards Eliza and tend to like her acting a lot more than other people (I don't see anything wrong with her in Dollhouse, despite most people having issues with her), but that's my opinion and I'm sticking to it.
What's interesting is that, according to a panel with Eliza, she and Sarah never talked about how to play each other -- they just watched a lot of film of each other and copied each other from there. Kind of an interesting way to do it, to me.
Interesting... you'd think the director would insist on some of that. Apparently for "The Killer in Me," Alyson Hannigan and Adam Busch both played all of the scenes and copied each other where appropriate.