MyDesigns works at the listing end. One design becomes many products across many marketplaces, with the publishing, the variants and the metadata handled for you. Its mockups exist because a listing needs images, not because anyone chose it for them.
Dynamic Mockups works at the image end. It renders artwork onto templates through an API, so images can be produced by your systems rather than by a person clicking. It has no opinion about where those images then go.
So the question is which end of your afternoon actually disappears. If you have five designs and publish each to forty products, publication is the cost and rendering is incidental. If you have four hundred designs and each needs one clean image, rendering is the cost and publishing is a button. Sellers who guess wrong end up automating the part that was already fast, which is why both tools attract reviews from people who describe them working perfectly and changing nothing.