This is a complicated question. Ultimately, the best product for the environment as a whole is the one you will buy once, care for well over time, and use for the longest period of time. A comforter can last for decades if covered with a duvet cover that’s washed more frequently than the comforter itself. The impact of most home goods and clothing is found in both how they are made but also how they’re cared for. Frequent washing and drying or dry cleaning uses significant resources and water.  As far as whether the industrial process that goes into converting used water bottles, bamboo or eucalyptus to material that can be used as a comforter filling is less impactful than growing cotton or raising sheep or ducks, that depends on many factors. These include where and how the bamboo or cotton is grown, where the sheep or duck waste goes and how they are fed, and how much energy it takes to process fibers into fabric or batting. The least-processed, and lowest-impact vegan material on this list is kapok, though it’s not as commonly found as recycled polyesters or lyocell is. So a kapok comforter likely uses less resources and water than one made from bamboo or down, but one thing you do know about a vegan comforter is that no animals were harmed or killed to make it. Environmental impact is a different and more complex question.