Both Shemini Atzeret/Simchat Torah as Shavuot are called 'Atzeret', both are about Matan Torah.
In what way are they comparable, and in which way are they different if both celebrate Matan Torah?
The Dubner Maggid was asked this question. This is a link to his moshol.
The bottom line is
The Maggid of Dubno explained that when the Torah was offered to the world, the nations raised many questions about its contents.
The Jewish people, however, were committed to keeping it, even before understanding everything it contained. "Na'aseh veNishma!" "We will observe and we will understand."
We celebrate this unquestioning acceptance of the Torah on Shavuot.
Yet their promise of total trust was incomplete. The mind can know and decide what the heart is not ready to accept. Only after living with Torah - only after completing a yearly cycle of studying the Torah and experiencing its teachings, as "Its ways are pleasant and all its paths are peace" (Proverbs 3:17) - only then is the Jewish people ready, on Simchat Torah, to express their true joy over God's precious gift.