To be honest, I don't like Caleb or The First as Buffy. However, I think my problem with Caleb is in the writing and not Nathan Fillion, but my problem with The First as Buffy is in both the writing, and the bland, unconvincing acting on Sarah's part. I rarely have problems with Sarah's acting but being the villain isn't her strongest suit.
I'm not a fan of Nathan Fillion, I think his acting can be very bland and I don't understand the hype he gets. Yes, he's great as Mal Reynolds, but everything else I've seen him in he's been average, as far as I'm concerned.
I'll give the edge to SMG, mainly because she's SMG and I think she's flawless, but it bothered me in the few eps where Buffy had a hoarse voice, like she had was suffering a cold, and The First suspiciously had the same affliction. If SMG really was ill during the end of that season, as many reports suggest, I think it could have been wiser to have another character play The First in those scenes, instead of putting double duty on SMG and ruining he continuity.
In fact, her performance as the First is a bit like that. Its very icy, and cocky. Think she did it really well. When you look at certain other moments during the series like Buffy acting out of character in When She was Bad or the classic example of her playing Faith in S4 then I think she really excels in the more sinister or unlikeable role.
Sarah's performance as The First was uninspired. She wasn't scary or cold, it was just the usual performance for that type of character. The bad writing didn't help. I only watched the S4 Faith episodes once and I wasn't impressed. Her flirtation with Spike or the bathroom scene didn't look one bit like Faith and I don't think Faith looked like Buffy either. I like Sarah in "When She Was Bad" but she wasn't unlikable or sinister. She was being colder than usual, while still being Buffy. SMG was very sexy and standoffish when she wasn't trying to be the cliched bad girl ('When She Was Bad', 'Bewitched, Bothered and Bewildered').
Well I didn't see anything wrong with the writing of her as the First either, but putting that aside I think she was much more subtle than you are giving her credit for. That's one of the hardest things for an actor play.
The point of the episode is not Buffy and Faith looking like each other. They've swapped bodies. Whoever said they looked like each other? They purposly look different because Faith is meant to be the anti-Buffy. The alternative Slayer.
In BB&B she is under a spell and it is played as comedy. Don't think you can really count that one.
I didn't say they had to physically look like each other. Faith needed to act and behave like Buffy and vice-versa and I think both actresses failed.