Yes, the difference is enormous -- and not subtle. Stone-ground grits and quick grits are not different grades of the same product. They are produced by fundamentally different milling processes that result in completely different textures, flavors, and cooking behaviors.
How They Are Made
Stone-ground grits are made by passing dried corn between two large stones that crack the kernel slowly, preserving the germ (the fat-containing center of the kernel) and the pericarp (the outer husk layer). Quick grits are made by steel roller milling, which runs hotter and faster, stripping away the germ entirely and grinding the remaining starch to a fine, uniform powder.
The Flavor Difference
The germ is where almost all of the corn's flavor lives. Stone-ground grits taste like corn -- sweet, complex, with a slight nuttiness. Quick grits taste like bland, neutral starch. The difference is not subtle. Side by side, most people who try both for the first time are genuinely surprised by how different they are.
Cooking Stone-Ground Grits
Stone-ground grits require 35-45 minutes of low, slow cooking with constant attention. Use a 4:1 ratio of liquid to grits. Bring the liquid to a boil, whisk in the grits, reduce to the lowest possible simmer, cover, and cook -- stirring every 5 minutes -- until the grits are completely tender and have absorbed all the liquid. Finish with butter, cream, and sharp cheddar.