...just updated the links section...

Quick thoughts on William/Spike and Angel/Angelus:

In seasons 5 & 6 we got the significance of Spike being called 'William' realigned. Prior to that, it had been used as a put-down by Angel/Angelus ("William, my boy"). It's Fool for Love which makes it clear that William was, in fact, his name before he was turned, and that Spike was a self-chosen vampire name (unlike Darla, Angelus and Dru who are all named by their Sires). William is his human self, in other words and something he wants to get away from. In season 6, especially, it is used to indicate his more human side: not only with Halfrek's entertaining double-take but most significantly, when Buffy finally breaks things off in As You Were. So "Spike aka William the Bloody" has become divided into two different signifiers: Spike, his self-created vampire image, and William, the human Bloody Awful Poet. After Angelus became Angel again, the characters immediately went back to using the 'human' name for him. In season 7, Buffy is - up till today because I'm not getting Never Leave Me until Thursday - still calling him Spike despite his ensoulment.

Other specualtion is about the different reactions to their ensoulment. As Spike points out, he did it to himself. This might be why, unlike Angel, who pretty quickly crawled back to Darla and went back to killing humans again*, Spike is initially so revolted when he realises he has been feeding off humans. Unlike Angel, who could feel he was a victim, Spike thought he was doing something that would make him "some kind of a man". Angel thought killing humans would bring him back his love, Spike knows it won't.

*although, as Darla points out, he only fed on "Rapists and murderers, thieves and scoundrels"