I vaguely remember someone who knew a lot about down posting that the primary different between various grades of down was how well the quills had been processed out. Therefore 600 more weight for same insulation as 700, 800 or higher. Also duck down could theoretically be processed almost as well as goose but typically is not. Some people just buy higher grades because the believe it will get them cleaner down, less sleeping with a wet duck experience. But I know very little about this so ask those that actually do not 2nd, 3rd hand information. There are also many other outdoor forums that discuss this and the down vendors are mostly very honest, unless you buy it super cheap via eBay or whatever from Hong Kong or some random duck farm in SE Asia, then you may be washing it and pulling out a mess of quills for hundreds of hours. LOL