Why do you feel it is different in any way? To me it is the exact same thing only different people. Angelus has no soul and Angel does and s6 Spike has no soul while season 7 Spike does. I personally fail to see any difference so I am curious what differences you see.
Two reasons, I believe, that there is the difficulty in distinguishing soulless Spike from ensouled Spike are that, first, there is no dynamic change in behavior. Spike with a soul is different, but the differences are more subtle, simply because Spike was an established character with a specific personality before getting his soul. Changing him drastically would be in effect creating a new character, something that Joss didn't want to do. It was easy with Angel, because we didn't know anything about Angelus, therefore he actually COULD BE a totally different character.
And, of course, there was no name change for the same reason. Everyone thought of him as Spike. If he came back in Season 7 suddenly wanting to be called William, it just wouldn't have seemed right to all of the Spike fans, and the Angel fans would be complaining about why their favorite has to be stuck with a name more suited for a Las Vegas stripper instead of his real name, Liam, while Spike was now going by a name that should have been followed by "the Conqueror."
(all but that last part, anyhow)
I have to day I dont think this is the answer at all, mainly cuz I think it's irrelevant to a discussion to combat a point with something in the show. Whatever happened in the show becomes what that character did so still has to be explained within the show.
There are indeed subtle differences, anyone that claims he's exactly the same is lying, but more important to the subtle differences is the glaring similarities, it's not that people dont see the differences, it's that they still see the similarities, they still see his lack of guilt and his view that getting his soul was enough for him to be forgiven.
In all theories about how the soul and vampires work, he doesn't fit. He doesn't feel a lot of guilt to the point where he wants to do good to make up for the many bad things he's done, so he seems to be able to separate himself from his pre-souled self. But he also takes credit for getting the soul - so how does that work, how can be believe he's changed and not be the same person anymore, but still take claim to what that person did as his own good deed?
[MENTION=7540]Kana[/MENTION] I like both your theories, I think I'd subscribe more to theory 2.
Theory one, Works in principle, that they are not the same person. But even in the evidence to support it, I could question the first quote and Wesleys, as both are from characters who got their training from the Watchers council, and I dont think they'd have actually given any research to the personalty of a vampire, or if they did they wouldn't be teaching that to their employees. Even Wesleys answer that it's an 'unholy beast' seems biblical, which there isn't a lot of in the series, it seems like something thats been ingrained in him
Same with both the Faith/Buffy and Angel/Marcuz body switch, unless I'm mistaken they are referred to as a body switch, not a soul switch, so I think it makes sense that the soul and memories get switched back and forth and each individual person keeps their own at all times.
I do agree that there is probably a bit of just calling someones life/conscience/spirit a soul and the soul being used as a general heading for all of it.
But at the same time, if they are completely separate I could understand why if Liam and William got back into the bodies and could remember everything from their respective views in the first person why they'd say "I did"
We know that the first reaction that both Angel and Spike had upon receiving their souls was an overpowering rush of guilt, so, yes, I would say that the soul, according to Whedon, at least, is primarily a conscience, and possibly the ability to feel the gentler emotions, like love, honor, and the like.
And while we know almost nothing about Harmony's life outside of Cordelia's shadow, it's impossible to say about how much she was loved at home, we do know that Liam was pretty much an outcast by his father, while William was deeply loved by and devoted to his mother. It's hard to make any kind of accurate assessment based on only those two individuals, but it leads me to believe that how close the human was with loved ones might have a significant influence in how the vampire turns out.
As for strength of personality, once more, outside of Cordelia's shadow, I don't have much to go on about Harmony's personality, other than that she appeared to be more of a follower than a leader, and William seemed to be a man who kept to himself and avoided being provoked into any sort of confrontation. Neither one would be what I would call dynamic, but that doesn't mean that they are especially weak.
Well what I see is both have weak personalities, I think the fact that Harmony has to be in Cordys shadow, or surrounded by cool people to build herself up, while Spike was in his mothers shadow. Both of them didn't have a distinct personalities, I guess what I'm theorising is that someone with a bigger personality when turned is a bigger evil, further away from the version of them that has a soul. Both Spike and Harmony dont have much of a personality and both dont cross over into big bad territory. They both flirt back and forward between bad and good, and maybe thats becasue there wasn't much personality to build on from their human version.
Both become personalities after they are sired, the vampire makes them who they are, forms more of them then there was to begin with, they develop becasue of the vampire. Whereas Liam has a personality (a crappy, drunk one) but he's got a big personality and it could be argued that he doesn't become the person he is becasue of the vampire, but Liam changes becasue of the effect the vampire has on him. Whereas William is who he is becasue of Spike, even to the point of keeping his the name, whihc on paper should remind him daily should remind him he tortured people with railroad spikes. He doesn't hate Spike becasue it is him, he'd be nobody without Spike. Liam hates Angelus becasue it's not him, but he's not the same Liam, he's Angel and doesn't feel he developed to that point becasue of Angelus, but in spite of Angelus and feels the need to correct the wrongs of Angelus.
This for me explains why Spike doesn't change his name, doesn't change his personality drastically or feel huge amounts of guilt. I'm off to ponder this more, may have just had a breakthrough in understanding Spike for myself.