Theoretically, an 8mm DA will finish out better than a longer stroke machine, assuming everything else, i.e., pads, polish, paint, remains the same. The same is true for DA sanders. Longer strokes = faster cut. Shorter strokes = a more refined and precise finish.
I don't really have the experience to validate that theory, as I have never used a long throw DA for polishing, but my logic suggests that to be true.
Similarly, forced rotation/orbit DAs may be faster than free-spinning DAs, especially when you need to use the pads edge, but based on my experience using a Rupes Millie (forced) and a Meguiar's MT300 (free spinning 8mm) on black vehicles, the MT300 finishes out better. Again, this is using the same pad and polish. Experimenting with different pads and polishes may close the gap somewhat, but if you are looking for the absolute best finish, I give the edge to the 8mm free-spinning DA.
