On any revolver with an even number of chambers, there will be one at the 6 o clock position, right where the cylinder notch has to be. This makes the amount of metal at the bottom of the notch thin. I am not sure if this has ever been a problem, as the area is quite small, but that is the concern. If there is an odd number of chambers, there will not be one at the 6 o clock position. Therefor, the notch falls between the chambers, where the metal is the thickest. I don't think there is any practical difference, unless there is a problem with the load, like the wrong powder or too much of the correct one. Both cylinders will have been tested to be safe at considerably more pressure than SAMI maximum. I did not let this matter be a deciding factor when I purchased my 3 in 686 Plus. I just went for the extra capacity. But I would have been happy with either.
Another consideration might be the weight of an empty cylinder versus a full one. With the seven shot, the difference is greater, and, while I don't notice the difference, I would think that an expert fast shooter would detect the change in the trigger pull as the cylinder is emptied while firing. It would take slightly less torque to move a lighter cylinder from rest then a full one, and this would be greater for the 7 shot, from first to last shot.