P4s and T34s have 50% more HP than Stugs in addition to a turret and much better anti-infantry.
Stugs are terrible in most situations. They are completely one-dimensional and sadly only useful for defense like a Pak40.
StuGs make a short work of T34 just as effective as SU-85 deal with P4s.
When you start seeing KV tanks, unless its KV-2, StuG is all you want if you're in T3.
While its not best idea, its definitely better to have StuGs against IS-2 then P4s that will simply loose without putting a fight, while StuGs can damage and at vet1 control IS-2.
You really underestimate StuGs. You just need to babysit them more, just like you have to with Su-85.