Something that hasn't really been considered here is thet in comparing Season 6 Spike with Season 7 Spike, we aren't quite seeing the difference between Spike and what I like to refer to as "Spikeus," meaning, obviously, pre-soul, evil Spike. In order to actually compare the two, you have to compare Season 2 and 3 Spikeus with Season 7 Spike. When you do that, the difference between the two is more noticeable. When he was first introduced in Season 2 and later in Season 3, and even on into Season 4, we see a totally different vampire than ensouled Spike from Season 7 on. With the introduction of the soul, the snarky personality from the earlier times is pretty much gone, leaving a somewhat quieter, certainly more introspective character. Season 5 of "Angel" sees the return of that to a certain extent, but, a least to me, almost all of that is aimed toward Angel, who undoubtedly seems to bring out the worst in Spike. In dealing with Gunn and Fred, and even later with Illyria, it doesn't seem to be there,
Basically, the main differences between Angelus and Spike as he is first portrayed is that Spike retains the human traits of loyalty and the ability to love, whereas Angelus doesn't, and Angelus seems to have a more cruel, sadistic nature, with no apparent feelings of loyalty. As first seen, Spike was capable of loving (Drusilla) and of forming at least rudimentary friendship (Joyce). By Angel's own admission, without a soul, he relished torturing and degrading his victims as much if not more than killing them. Spike, on the other hand, claimed to be "all about the kill." Both vampires were monsters, predators, and killers, but each was unique from the other. Possibly, even probably, Angelus was the more monstrous of the two, but both were, definitely, monsters.