I never understood the explanation for why Spike could hit Buffy. She didn't come back wrong, but she had changed on a molecular level? What? Deep tropical tan? Talk sense, Tara.
I think it means that, raising Buffy from the grave, putting her soul back into her rotting corpse, regenerating that corpse, the magic that was used to tie it all together, all of that changed Buffy's makeup on a genetic level, meaning that she didn't register as human to the chip anymore. It didn't change or affect her anymore severely than the equivalent of a deep suntan. Hence Tara's analogy.
As for the OP, I think it all depends on just how much a lack of soul changed Buffy. The soul has different meanings here than other shows (Supernatural springs immediately to mind) so I'm think that Buffy would have been very, very evil, on top of having super powers.
But I believe the question has already been answered in the show, by both vampires.
Angel is always willing to make the hard choices, killing Connor, killing Cordelia, killing Darla, Drusilla, whoever, he does it because he knows it's the right thing to do.
Spike knows that Willow and Xander and the Scoobies kept him out of the loop on bringing Buffy back because, if she came back wrong, Spike wouldn't have allowed them to take her away again. He would have done everything to keep whatever version of Buffy came back because, to him, she would have still been Buffy. Even if she was evil.