The Mezzanine

The way I see it, Angelus seeks the pain of others (specifically weak and innocent humans while soulless) in order to find pleasure, while Spike seeks pleasure and most often finds it in the pain of others (while soulless and unchipped). The outcome is the same but I think the outlook behind it is slightly different: Angelus is fairly stable in his desires, but we see Spike not only getting sadistic pleasure, but also masochistic pleasure (when Dru cuts his face in School Hard), a sense of comfort from luxurious food (with Joyce in Lovers Walk) and enjoying violence for the sake of violence (with the mobs and the slayers in the FFL flashbacks, where there was risk of pain to himself as much as others). He's more flexible about his source of fulfilment, which I would say allows him to adapt to chipped life and get on the path to good much more readily than Angelus would have been able to.

And I would say Angelus is an artist - a massively Baroque one, who missed out on the main movement by being born too late and living over in Ireland. I would also say he is seriously, seriously warped and evil. But I don't think the two are incompatible.