Substance is much better for a production or studio environment, you can have master substances for an entire project or game, and update every asset that uses them at once, very quickly and seamlessly. Linked below is a really good GDC talk about how they used it for remaking Halo 2 art assets. Substance is definitely smoother and nicer to work with, Quixel is a bit clunky because of being tied to Photoshop. Substance Designer is also much better for anything involving custom tiling textures. Although I think Quixel is a bit better for a single artist just wanting to throw a few smart materials on a mesh, tweak the values, and call it done. Substance is being used a lot by studios in the game industry.
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