Angelic Slayer
Daydream Believer
Wow, that's a great question. This is my understanding of the situation.....
I think that when they talk about someone having a soul in the Buffyverse they're referring to the essence of the person. With a soul comes a conscience aswell as certain abilities to feel things (such as love, although I'm not sure I agree with that it seems to be the way it is written).
In Angel's case, Angel's soul is in control of the body. But you have to remember that he is still a demon. He would still have the impulses for violence and to cause pain that Angelus (his un-souled self) would. He obviously has to fight these impulses, and since these are impulses of the demon part of him it would seem like he is fighting off Angelus (since Angelus is really the demon part of him). Both Angel and Angelus share eachother's memories aswell as a body. This would make it difficult to invision the line that would separate them. So they only thing that really would be the difference is that Angel has a soul. Technically, Angel COULD do everything Angelus has done but his conscience prevents him from doing so. Even though Angel may be aware that with his soul he is, in a way, a separate being entierly from Angelus, he has the memories of what Angelus has done (in his body), which would cause him to still feel guilt for the crimes that were committed.
As far as the soul itself, I guess it would be the soul of Liam. However because he holds the memories and guilt of life during/after Angelus was in control, he would be an almost completly different person that he was before he was turned.
The differences in the characters of Angel and Angelus are like two extremes. Angelus is loud, Angel is typically less talktive. Angelus kills for pleasure, Angel rarely if ever kills humans at all. Angelus wants to hurt the people around him, Angel wants to protect them. It also seems like Angelus changed after Angel was restored. When you compare the flashbacks of Angelus (originally) to the Angelus in say, season 2 of Buffy, the newer Angelus seems more angry and intent on hurting one person (Buffy). This is probably because he hated how Buffy had made his ensouled-self feel and even just the fact that he was forced to become more human (and Angel's actions after that happened). This anger would cause a change in the demon's personality.
As far as Spike goes, he is/was a completly different vampire/human than Angel. He also has had different experiences and feelings than Angel. So it only makes sense that the difference between him with a soul/ without a soul would be different from Angel/Angelus. When he first got his soul he changed a lot, at first the change was more noticable, but after Sunnydale was destroyed and he came to Angel's show his personality changed once again. But remember, personality changes can occur in humans all the time. The difference between Spike and Souled-Spike may be less extreme but it's still there.
In addition, (as James Marsters once said) ever since Spike was added as a permanent cast member on BTVS, they have been slowly humanizing him. First the chip (forcing him to fight against evil and drink pig's blood), then he started helping Buffy (for cash, later for no charge), then he became obsessed with Buffy (which came as close to love as it could have concidering at this time Spike didn't have a soul), and eventually he actually got his soul and regained a bit more of his humanity. Because of this "humanization process" and the gradual change that accompanied it, Spike wouldn't seem all that different. That and his distaste for Angel would just make him more like his "old-self". I think that Spike kind of created a new "way of life" for himself that wasn't quite like his old/evil one or the one he had in season 7 of BTVS.
Overall, I think that a vampire being ensouled (and the personality change that would occur) would change from vampire to vampire. It would be based so many things; their human self, the things their demon did, how they were ensouled, the people around them, even the time (date-wise) in which they were ensouled could have an effect on the way they are. On both Buffy and Angel we saw that humans aren't perfect either and that some will kill (even with a soul), or worse, and in this case (if they were changed) there may not be a real difference at all.
I think that when they talk about someone having a soul in the Buffyverse they're referring to the essence of the person. With a soul comes a conscience aswell as certain abilities to feel things (such as love, although I'm not sure I agree with that it seems to be the way it is written).
In Angel's case, Angel's soul is in control of the body. But you have to remember that he is still a demon. He would still have the impulses for violence and to cause pain that Angelus (his un-souled self) would. He obviously has to fight these impulses, and since these are impulses of the demon part of him it would seem like he is fighting off Angelus (since Angelus is really the demon part of him). Both Angel and Angelus share eachother's memories aswell as a body. This would make it difficult to invision the line that would separate them. So they only thing that really would be the difference is that Angel has a soul. Technically, Angel COULD do everything Angelus has done but his conscience prevents him from doing so. Even though Angel may be aware that with his soul he is, in a way, a separate being entierly from Angelus, he has the memories of what Angelus has done (in his body), which would cause him to still feel guilt for the crimes that were committed.
As far as the soul itself, I guess it would be the soul of Liam. However because he holds the memories and guilt of life during/after Angelus was in control, he would be an almost completly different person that he was before he was turned.
The differences in the characters of Angel and Angelus are like two extremes. Angelus is loud, Angel is typically less talktive. Angelus kills for pleasure, Angel rarely if ever kills humans at all. Angelus wants to hurt the people around him, Angel wants to protect them. It also seems like Angelus changed after Angel was restored. When you compare the flashbacks of Angelus (originally) to the Angelus in say, season 2 of Buffy, the newer Angelus seems more angry and intent on hurting one person (Buffy). This is probably because he hated how Buffy had made his ensouled-self feel and even just the fact that he was forced to become more human (and Angel's actions after that happened). This anger would cause a change in the demon's personality.
As far as Spike goes, he is/was a completly different vampire/human than Angel. He also has had different experiences and feelings than Angel. So it only makes sense that the difference between him with a soul/ without a soul would be different from Angel/Angelus. When he first got his soul he changed a lot, at first the change was more noticable, but after Sunnydale was destroyed and he came to Angel's show his personality changed once again. But remember, personality changes can occur in humans all the time. The difference between Spike and Souled-Spike may be less extreme but it's still there.
In addition, (as James Marsters once said) ever since Spike was added as a permanent cast member on BTVS, they have been slowly humanizing him. First the chip (forcing him to fight against evil and drink pig's blood), then he started helping Buffy (for cash, later for no charge), then he became obsessed with Buffy (which came as close to love as it could have concidering at this time Spike didn't have a soul), and eventually he actually got his soul and regained a bit more of his humanity. Because of this "humanization process" and the gradual change that accompanied it, Spike wouldn't seem all that different. That and his distaste for Angel would just make him more like his "old-self". I think that Spike kind of created a new "way of life" for himself that wasn't quite like his old/evil one or the one he had in season 7 of BTVS.
Overall, I think that a vampire being ensouled (and the personality change that would occur) would change from vampire to vampire. It would be based so many things; their human self, the things their demon did, how they were ensouled, the people around them, even the time (date-wise) in which they were ensouled could have an effect on the way they are. On both Buffy and Angel we saw that humans aren't perfect either and that some will kill (even with a soul), or worse, and in this case (if they were changed) there may not be a real difference at all.