There is no fundamental difference between the 6-shot and the 7-shot other than the number of holes in the cylinder.
Some shooters believe that the 7-shot cylinder is actually stronger because the cylinder stop notches are between the cylinders where the metal is thicker.
The 6-shot is slightly more common with the old-style design features that traditionalists favor- i.e. forged rather than MIM lockwork, and no internal lock. The Plus actually was
produced with these features, just not for very long, so collectors covet these particular guns.