I'm assuming that people who say bamboo requires toxic manufacturing process, are right. I don't know that for a fact. That is a concern for me if it is true. Unfortunately, cotton may not require nasty chemicals during processing but it definitely requires nasty chemicals in growing it. The defoliants they spray when getting ready to harvest it are not something most of us want to be around. If it is organic, I assume that is not true, they must have another way to harvest it. Polyester is a petroleum product and its processing can not be all that "clean" either. I would think wool is probably the least offensive to our eco-system. It is easy to get on the bandwagon about the earth-unfriendliness of bamboo, but the other choices aren't all that earth friendly either. Because bamboo grows like a weed, it probably doesn't require the toxic fertilizer's that cotton does. Because of all these things, and because I love the feel of it, wool would be my choice, but it is expensive and I can't afford to use it all the time. We should let the batting company's know that we want them to find "kinder" ways to produce our battings. Cindy Needham, a well known quilting teacher, suggests using a layer of thin cotton (like Quilters Dream request) and top it with a half layer of wool. She says wool will naturally pull apart into two thin layers. She calls it "gourmet batting" As far as the original question goes, I would answer "it depends". I wouldn't want it to be much heavier, just slightly loftier. I would think a wool, cotton blend or wool, bamboo blend would be great.