I've read Clara's reviews of both these yarns, and the related posts, and am wondering what the main differences and similarities are. (Since the Quince write-up is a preview, not a review, there isn't any knitting/washing/wearing info on which to bade a comparison.)
The biggest difference is that the yarns are spun at two different mills using two completely different techniques. Quince is spun worsted and Shelter is spun woolen - so it's the difference between a bread made from white flour and one made from rye or whole wheat. Quince yarns are also dyed solid colors, while Shelter is dyed in the wool so that the colors are blended - in fiber form - before ever spun. Hope that helps!
Having projects with both OTN at the moment, I'll add that Shelter is more of a woolly, rustic yarn while Quince is smoother and more refined. Like the difference between a tweed jacket you'd wear outside vs. a dressy blazer you'd wear indoors. Both are lovely but they are nothing alike despite the similar fiber content.