As others have said, they are similar. The difference as I see it is that the Dormition concept seems to emphasize that Mary experienced bodily death before being resurrected and assumed into Heaven by Jesus. The Catholic Church does not definitively address the question of whether Mary experienced bodily death or not before she was assumed into Heaven. We are free to believe that she did, or free to believe that she did not. The Catechism just says “when the course of her early life was finished” she was assumed body and soul into Heaven.
I went to two Assumption Masses this year. At one, the priest stated in the homily that Mary never experienced death, that she was spared death because she did not have original sin. At the other, a different priest stated that Mary died and was assumed into Heaven after her death. Many Western Catholic articles and books also will make a statement that she died or she didn’t die.
It’s a moot point to me because the important thing is that she was assumed body and soul into Heaven. Whether this happened to her instead of dying, or one second before bodily death, or one second after bodily death, or after she’d spent three days on a bier or in her tomb, is not important to me, especially since the Latin Church doesn’t take a position. (I presume that if she did experience bodily death, her body did not decay at all, which would be in keeping with her being the holiest of all humans, since incorruptibility seems to happen frequently with holy saints.)
I found it interesting when I toured the Holy Land with a Roman Catholic group, the Church of the Dormition was not on the tour. I thought it was an odd thing to leave out. I wondered if they skipped it because of the confusion about the death or non-death of Mary.