During Chazaras HaShatz of Shemoneh Esrei the beginning of "Kedusha" is different in Nusach Ashkanaz than it is in Nusach Sephard. What is the reason for the difference and where does each nusach come from?
Half an answer. I'm guessing the question arises since both Nusach'es are of Ashkanaz origin. A quick look at what is Nusach Sepharad [source]
Nusach Sefard, or 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 the Ari. To this end it has incorporated the wording of Nusach Edot haMizrach, the prayer book of Sephardi Jews, into certain prayers.
And one of those prayers is Kedusha.
As to why HaAri HaKadosh preferred Nusach Edot haMizrach that's a different question (something to do with kabala as mentioned).
And why do Ashkenaz and Edot HaMizrach have a different Nusach? I searched that a bit. Even after Churban Ba'it Sheni there were several Nusach'es. Certain groups adopted certain Nusach'es. For example, G'eonim of Israel or Babel. From there, a more common Nusach stayed and when jews moved to new places they brought their Nusach along. It ended up that most jews who reached the countires of Edot Hamizrach followed 'that' Nusach and the same happened to the jews who came to Askenaz lands.