Between the buffing and grinding wheels and a couple of other powerful shop tools, there are some scars on paneling and even a couple on me from having work grabbed out of my hands. My face mask split when a chunk of a new grinding wheel came loose.

The significant difference between the wheels is the surface speed. A 6" at 3600 rpm has a surface speed of 5655 while an 8" has a surface speed of 7540 FEET PER MINUTE or 33+% faster - which means anything grabbed out of your hands will be moving a heluva lot faster/hit harder with the greater diameter (I use 8" wheels)