I have a Ray Jardine quilt that is supposed to be rated roughly the same as two of my down quilts (Jacks R Better). It is not as warm and is twice as bulky. It compresses to about basketball sized where the down quilts get down to cantalope size. The synthetic is probably 6 oz heavier. 800 fp down wins....

There is also quite a noticeable difference between 800 and 600 fp down, when you look at two jackets compressed side by side. I have a Marmot jacket that almost fits into one of its own pockets.
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