Q. What is the correct pronunciation of the first two words of Kaddish?
yisgadEl viyiskadAYsh or yisgadAl viyiskadAsh?

A. Poskim disagree as to the correct pronunciation of the first two words of kaddish. Mishna Berura (56: 2) quoting P’ri Megodim asserts that one should pronounce the first two words of kaddish with a tzeirei under the daled since it is the Hebrew and not the Aramaic version of the posuk in Yechezkel (38: 23) “And I will reveal Myself in My greatness and in My holiness.” So too is the opinion of the HaGra quoting Rashi (Ma’ase Rav 54). Mateh Ephraim (Kadish 20), Pische Olam (56: 1), Aruch Hashulchan (56: 3), Orchos Chaim (ibid. :4) quoting Eishel Avrohom and others.
However, other Poskim maintain that the correct pronunciation is with a patach (Mekor Chaim, Ya’avetz in his sidur, Ba’al Hatania in his sidur, et. al.). The fact that most sidurim from olden times and those printed today spell the words with a patach, speaks volumes. (See Kuntres Ohr Yisroel 48 p. 156. The article also explains the historical aspects of the change from the ancient patach to tzeirei)). Patach pronunciation is also the most accepted tradition by Ashkenazic and Sepharadic communities.
Horav Shlomo Miller’s Shlit’a opinion likewise is that the patach pronunciation is the more grammatically correct.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a