If Eliza had been chosen to play the title role in the series, which is, I assume, what the subject is, regardless of the character's name, she certainly wouldn't have been the Buffy that we all know and love. (Someone, feel free to say, "Well, DU-U-U-U-H," any time, now) I won't say that Eliza would be incapable of playing the role that way, but I certainly don't think that she would be comfortable doing so on any sort of a long term basis. Just about every role that I have ever seen Eliza in has had one thing in common. She's a tough girl, street smart, ans self capable. That's not the way that Sarah played Buffy. Sarah's Buffy was more strong than tough, and more sophisticated (in a teenage girl sort of way) than street smart. Sarah's character dressed for fashion, with an eye on practicality, whereas Eliza's character, whatever she plays, usually dresses as fashionably as is practical for what she is doing.
It's just a case of two different actors creating the same character within the field (I won't say limit) of her comfort and capability.
