Thanks mepal.
I saw the following in the Tallelai Oros on Tfilla (4th chelek) quoting a perush I had never heard of (also one I cannot pronounce)
Why in Nusach Ashkanaz, do we say ????? ????? after ?????, but in nusach sfard we say ?????? ??????, like we do in our silent Shmoneh Esrei?
In Nusach Ashkenaz we say
????????? ??? ??????? ????????. ??????? ????????????????? ???? ????????? ?????, we compare our prayer to the prayer of the malachim. We claim that we are mekadesh Hashem the same way the angels do.
But man’s time on this earth is limited while the angels last for all eternity, how can we make such a claim?
Therefore we immediately follow kedusha with ????? ????? ??????? ????????? ????????? ???????? ???????????? ??????????, generation after generation, for all eternity we praise Hashem. Now we can truly claim to praise Hashem in the same way the Malachim do.
In Nusach Sfard, However, we say ??????? ???? ??? ?????? ?????, we do not compare our praise of Hashem to the praise offered by the malachim, therefore we can say ?????? ?????? in Chazaras HaShatz just like we say it in our silent shmoneh esrei since we do not have the reason to switch to ????? ????? like those who daven Ashkanaz