I think that in Joss's canon, some vampires turn out very human, and others are monsters. We see the bookish Dalton and the sweet William and the still very Harmony Harmony. So, I do think that the personality of the original human affect the demon a lot. Spike knew love as a human... he knew deep and lasting love, and so his demon has access to those memories. For Angelus, his father hated him, and that family love twisted into guilt and hatred. I think that makes it a lot harder for Angelus. However, Angelus was in there while Angel loved Xander, so I do think he can change. I just don't think it's going to be easy for him.