Not exactly scientific. Just a photo and a soak test.
Both are calcined diatomaceous earth.
Both packages list less than 1% Crystalline Silica.
Napa lists less than 1% quartz.
Both pass a 24 hour soak test in that neither softened nor crumbled easily. I did not freeze test them since my soil never freezes.
I like the uniformity of the CarQuest product much, much more than the Napa. Both examples are un-sifted. Napa MUST be sifted before use in a '5-1-1' mix or gritty 1-1-1 mix. (Check this page out if you have never heard of the 5-1-1 or gritty mixes)