Depends on what you want them for and how you like to work. Trying to make original textures for your objects or other aspects of your scene? Substance is the likely best choice. Want to add objects, ground cover, or other real-world-derived textures and visual detail to your scenes (probably not mapped to an object)? Then probably Quixel. There is some overlap, especially in e.g. Quixel Mixer vs. Substance, but that is generally a fair distinction. Quixel Mixer is also primarily layer and slider-based, whereas Substance also has a heavy node component (at least for advanced stuff), so depending on how you prefer to work, you may like one vs. the other. They both have nice interactive previews and are reasonably easy to use though.
- Oshyan