giphy.com/gifs/spike-xgZUFqQCGq7II
But seriously - Spike's behavior in season 7 (not including when he's being controlled by the First, obviously) definitely shows a conscience. Just because he isn't being all "I want forgiveness" like Angel, doesn't mean that he has no conscience.
Angel clearly states that his greatest desire is "forgiveness" (Literally - "What do you want, Angel?" "I want... Forgiveness"). And he isn't talking about personal forgiveness - he's talking about cosmic forgiveness, about the universe forgiving him for his crimes. And that's a problem for two reasons:
1) Angel isn't even responsible for all the people that he killed as Angelus, so his quest for forgiveness is pointless. I mean, it's good that he wants to help people, but he's doing it for the wrong reasons.
2) It makes Angel OBSESSIVE. Angel completely devotes himself to his quest for "redemption" (more like atonement, really), and while that leads him to help a lot of people, that also leads him to do some terrible things. Angel killed Drogyn while seeking redemption. Angel got L.A sent to hell while he was seeking redemption. Angel hurt many people during his "beige" phase in season 2.
That's the problem with Angel's approach to "redemption". A problem that ensouled-Spike doesn't share, thankfully, since he's not obsessive about redemption. For people like Drogyn, Spike was actually a much better champion than Angel.
How did this turn into an Angel versus Spike war again? I swear, it feels like EVERY thread turns into this at some point...
The crucial difference is that Faith actually had a redemption arc (as in, she went from a bad guy to a good guy), whereas Angel just had a switch (the soul) that turned him from good to bad to good. Faith CHOSE to do evil, and then she CHOSE to change back. Angel never made those choices. That's why Faith is a lot more like Anya than like Angel, in terms of redemption. Unlike Angel, Faith is actually GUILTY of the bad things she did. Unlike Angel, Faith actually has a reason to feel bad for what she did. And unlike Angel, Faith actually made the choice to change after she lost herself in darkness. Angel had no agency in any of these things. Faith and Anya and Andrew did.
Another difference is that unlike Angel, Faith isn't obsessed with redemption. Faith didn't join the scoobies in season 7 for "redemption", or to punish herself, she did it because she was needed ("The way Willow tells it, that's where I'm needed"). Faith didn't save Angel for redemption, she did it for personal loyalty ("Angel's the only one who never gave up on me, there's no way I'm giving up on him"). Unlike Angel, Faith isn't obsessed with some cosmic forgiveness - she just wants to do good, and to feel good (and doing good makes her feel good, as Faith learned in 'Who Are You'), and to not hurt people anymore. That's why Faith doesn't end up doing things like killing Drogyn - she doesn't devote herself to redemption as some exalted ideal, she just tries to live her life as a good person and a good slayer. Which is far closer to souled-Spike's philosophy than to Angel's.
In seasons 5 & 6, sure. But in season 7? After he gets a soul, Spike doesn't kill people because he has a conscience, not to get into Buffy's pants. The show was clear about it.
All of these things are found in both Anya and Faith.
1) Like Faith, Anya buried her conscience. For a much longer period, sure, but it's still the same thing.
2) Like Faith, Anya made a CHOICE to kill.
3) Like Faith, Anya made a choice to STOP killing.
4) Like Faith, Anya chose to suffer for her crimes - she asked D'Hoffryn to kill her in exchange for the lives of the people she killed.
So, I don't see the difference.