So I was wondering what differences there are between Nusach Sefard, the traditional Chassidic nusach and Nusach Ha-Arizal, the Chabadnik nusach arranged by the Alter Rebbe (Rabbi Schneur Zalman of Liadi). Does anyone know the difference?
Nusach Sefard, Nusach Sepharad, or Nusach Sfard is the name for various forms of the Jewish siddurim, designed to reconcile Ashkenazi customs (Hebrew: מנהג "Custom", pl. minhagim) with the kabbalistic customs of Isaac Luria.
Technically speaking, the Alter Rebbe's siddur is a form of nuschach sefard.
Nusach Ari is a variant of Nusach Sefard, used by Chabad Hasidim.
Practically speaking, no one calls it Nusach sephard (even wikipedia separates it from Nusach sefard). My guess would be that's because of the separation between chabad and other Chassidim. Either way, Chabad definitely views it as separate nuschaos. I can't speak for the other Chassidim though, and I haven't asked them. But they're both founded from the same sources, and share many similarities.
My understanding is that Nusach Sfard is the nusach that was created by those who wished to incorporate the customs of the Ari.
Many different versions of Nusach Sfard have existed and because of this there has been many attempts by various Chassidishe groups to reconcile the differences amongst versions into one standard version for their group.
The most prominent of these redacted versions is the Chabad version of Nusach Sfard which they call Nusach AriZaL. This was the Alter Rebbe's attempt to standardise Nusach Sfard for his Chassidim.
All of the versions of Nusach Sfard, including Nusach AriZaL, are quite similar. I myself use an Artscroll Nusach Sfard siddur and daaven by a Chabad shule and I get by with very little difficulty whatsoever.