I think Liam seems vulnerable in a way that I don't see in either Angel or Angelus. Angelus has this whole "I'm better than you" vibe going, which, given he's Angelus, I rather expect. But Angel, at least in season 1, does as well. When he and Buffy interact, in s1, his goofy boyfriend smirk seems to have a hint of patronizing a-hole. At least that's what I see. Liam's more open than his other two personas.