What's the difference between the 3M diamond disk kits and the smoothing disk kits?
The 3M diamond disk kits with velcro backing, Dual Loc, and Roloc backing and are made from 3M's 6002J electroplated diamond material. They range in grit from 60, 120, 200, 400 and 800 grit. Being electroplated diamonds, even at 800 grit the surface you'll get from them will be rather rough. Not nearly clean enough to polish from.
To move the surface further along, you'll need the diamond smoothing kit.
These kits contain resin bonded diamond disks where the diamond is encapsulated in a resin and slowly spins in the resin as your grind with them. This leaves a much "smoother", finer finish than a comparable electroplated diamond. The smoothing kits contain 220, 325 and 600 grit resin bonded diamond disks for achieving a pre-polish finish and cerium impregnated disks for achieving a final polish.