The 7-shooters are possibly stronger than the 6-shooters because the weakest point in the cylinder is the locking cutout, which is directly adjacent to the chamber on the 6-shooter, but on a 7-shooter, the cutout is offset between the chambers. This is a superior design. Not that the 6-shooter isn't strong enough, though.
There is no reason to believe that the 7-shooter is a compromise in strength. The metal around the chambers in the 6 shooter has the thickness it has to compensate for the placement of the cutout notch. So with the cutout notch being offset, the 7-shooter cylinder is perfectly suitable for the application because of the offset cutout notch.
Cylinder wall thickness on the 7-shooters is still a tad thicker than it is on K-frame 6-shot .357 cylinders. And the steel may well be stronger as well. I have no worries at all that the 7-shot models are strength compromised.