After many more days of usage, I finally understand what @PsychDrummer means.
In summary, the Supcase attracts a lot more dust than the Tech21. Especially on the screen edges and speaker holes. I had to use a brush to clean it out every other day.
I'm not sure why, but I couldn't stand using the Supcase more than a few days, then I had to switch back to the Tech21. It just feels right for me over those two cases