The StuG was built on the Panzer III chassis and hull, so from the sides the two were equivalently armored. The later models of Panzer III and StuG had side-skirt armor largely for protection against anti-tank rifles; the panels tended to destabilize the projectiles and reduce their effectiveness. I'm not aware of specific combat reports detailing 14.5mm shots penetrating side-skirted tanks, but that is not to say it did not or could not happen. In urban environments and forests the skirts had a pronounced tendency to get snagged on things and come off. Given that this terrain was the optimal terrain for bold PTRS teams, the presence of skirts might have been mostly moot.