(The Ostwind should have more armor and the PIV should have more health)
640 is the standard health for medium tanks, and 160 is the standard for damage. Giving it even a single health point more would increase the Shots to Kill from 4 to 5, making it even more superior to other stock tanks.
The StuG being able to take on infantry and armor like the PIV but lacking a turret would make it function like an actual assault gun which is supposed to attack enemy armor and entrenched infantry. You would get a PIV if you need more mobile AI and AT. For instance I would want a StuG if I'm attacking an enemies fortified position, but a PIV If I need to support a more mobile force.
The problem with the dual AI / AT StuG is that it doesn't serve an unfulfilled purpose. The Ostwind is hard AI (also AA), and the Panzer IV is soft AT / soft AI, but the StuG is also soft AT / soft AI. The StuG doesn't really serve much of a role other than being cheaper than a Panzer IV but lacks durability.
Personally, I'd rather see the StuG increased to T-34/76 price and AI lowered, in exchange for armour, health, range, and penetration. HE / AP swap would be fun, but that'd steal the Sherman's thunder.