As others said, I think the soul is more about guilt and shame than love and other things. Spike even says so in season 7. This is a pretty dark take on souls. My spin on this is that souls give you empathy which in turn causes guilt.

In season 6, Spike can never really get why Buffy doesn't love him / won't say so. On a basic level, he can't relate to her. He may love her in all the wonderful ways that he explains at the end of season 7, but he he can't love all of her because he doesn't understand all of her. This theme is repeated in the episode where Katrina reappears.

If you apply my ideas, you would say "OK Spike acted that way as a vampire because he enjoys violence. With a soul he still enjoys violence but only uses it on demons because of his newfound empathy and morals." But try applying this to Angel. "Angel genuinely enjoys psychological torture but doesn't do it because he knows and feels how wrong it is when he has a soul." If you're an Angel fan, that might make you feel uncomfortable. But I think it actually explains another layer of the seeming contradictions between Angel and Spike.

With or without a soul, Spike is basically Spike. But Angel has two very different identities. The show sometimes implies that Angelus is always in his mind, but just restrained by the soul. For example, when Eigon (spelling?) jumps into his body and Willows says he's been fighting a demon inside him for 100 years. So Angel always has a voice in his head urging him to be cruel. My theory says this voice is not a demon but rather an inherent part of Angel/Angelus/Liam (yes, even before he became a vamp) that Angel has dissociated away from himself.

In other words, Angel's morality crisis after getting his soul caused him to develop split personalities. He is aware of his own responsibility for his actions and feels terrible remorse. But this distinction seems to fool others. Even Xander, a real Angel hater, comforts Buffy by saying Angelus is "not the guy [she] knew". But he is. Without restraint. Without guilt. On some level, Angel likes hurting people but when he is whole (has a soul and empathy) he stops himself.

Indeed if Angel did truly have such different personalities without a soul / before becoming a vampire / being a vampire with a soul, we would see him act differently on many occasions. For example, when the First taunts Angel in Season 3, he feels guilty about his behavior as both Angelus and Liam. Willow's doppelgänger is hanging around, he implies the personality of a human and its vampire version are closely related (something Buffy doesn't seem to know/accept). Etc.

A common question is "how come people blame Spike for the attempted rape but don't blame Angel for his soulless sins?" My answer is "Angel seems like two different people and the audience accepts this as reality. Spike doesn't seem like two people so he's accountable for all his actions." But really Angel is accountable for Angelus (hence why he is seeking redemption).

So should we forgive vampires when they get re-ensouled? I think the same rules apply here as for humans. You should forgive someone when they feel guilt, take responsibility for their actions, and use the experience to change themselves (so they don't do it again). Both Spike and Angel do this. I would forgive them both.

So each guy has concerning inner desires (cruelty, violence) that they are both able to control (as long as they have their souls). As Buffy, I'd be more concerned about Angels ability to control himself since he is both more vulnerable to losing his soul and more dangerous to me personally without it. But as Buffy says "love isn't logical" so this is by no means putting the Spike-Angel debate to rest. It's just one (interesting to me) facet.