Usually Spike's fans complain that Angel's quest for redemption and atonement is too over the top, because if souled vampires are not responsible for the evil they did when soulless then they shouldn't feel so guilty.

On the other hand, Angel's fans complain that souled Spike is too callous and uncaring in regards of his evil deeds, because even if he is not responible for them he still remembers them and should feel more guilty.

Many believe that the reason is in the different way they get souled: Angel by force, Spike by choice. Also there is the different personality. And there is Buffy's positive influence that was missing for the freshly souled Angel.

Me, I don't know how much of both complains are valid because the first thing Angel did when souled was justify himself to the gipsy who cursed him saying "It wasn't me" and then he kept feeding on humans and tried to go back with Darla. While Spike spent many days in a basement trying to scratch away his soul and then sacrificed himself to save the world. So it's quite the opposite some say.

But that's on BTVS. On ATS Angel's story is definetely about his quest for redemption while Spike's attitude in that single last season is a 'I don't care' one.
So let's say it's like that, I agree with the fandom's explanations I wrote above but I think there is also another reason, that lies in the knowledge the two vampires have of themselves and of their capacity to be good or evil. One quote from Angel strikes me with the truth of this: "It's not the demon in me that needs killing. It's the man."
No one knows Angel better than himself and he knows that he is capable of doing very bad things, that deep inside he is bad, regardless of his demon. Because of those 100 years wondering about without doing anything good and just looking bored, I don't really believe that Angel feels guilty for what he did when soulless, I think he feels guilty because... he doesn't feel guilty, because rationally he thinks he should and he starts thinking so only long after he meets Buffy, when he first-hand understands how people and soul should work. Good people feel guilty if they do something bad. But his issue is that his soul doesn't work as it should so he has to 'work harder', much like soulless Harmony said. That's why imo his guilt is so over the top, it's because it's made up, it's his mind trying to teach his soul how it should work.

Spike is different. He is inerently good, he was good when human and is always good when when souled. Oh, he can be a dick sometime and some may find his attitude annoying but since souled he has never done anything even remotely evil of his free will. And we are talking of 2 season in the shows and 3 in the comics, so it's safe to say that he truly is a good guy. And when he gets his soul back he needs some time to adjust but then he is able to go on without letting the guilt burden him so much. Because just like Angel, Spike knows himself, he knows that 'he' would never do such horrible things and this helps him distance himself for what he did when soulless. He is the same person but at the same time... he doesn't feel the same.