Guterman has always been thinner, but it's strong which is my criteria and now that I've started quilting, there is some value in a thinner thread taking up less space in the seam after it's stitched. I am using old C&C on the top on the 15 clone and I see the difference when I press the seam. I also like a thinner thread in the bobbin on the 301 because it doesn't hold much thread to start with.