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Small update.
Out of interest, I've calculated the probabilities for the following scenario:
3 StuGs or 2 SU85s (reasoning that both vehicles are casemates, their cost in fuel of the setup is exactly the same plus they are the most similar in ROF) shoot at a 300 armor target (e.g. Tiger/Pershing) at range 50.
Both groups are allowed to shoot twice, making 4 and 6 shots for SUs and StuGs, respectively.
So, small summary of probabilities:
Failure to stop the enemy, defined as the units manage to pen only 0-2 times
SU85s = 27,1%
StuGs = 22,8%
Also chances for 0 or one hit are comparable, if not negligible anyway.
Chances that one group fares better than the other, defined as they get at least one more pen than the other group:
SU85s better: 27,0%
StuGs better: 47,6%
both the same: 25,4%
Chances that one group fares MUCH better than the other, defined as they get at least TWO more pens than the other group:
SU85s much better: 9,6%
StuGs much better: 23,4%
groups perform "similar" (meaning the pens do not differ by more than 1): 67,0%
I obviously have to leave a lot out of this, for example that it is harder to micro 3 StuGs than 2 SUs, the 10 range difference that would allow the SUs to shoot a bit earlier, veterancy, but on the other hand I also neglegted accuracy and scatter both in which the StuG fares better as well as ROF were the StuG leads as well by 0,4s as well as defensive values such as armor and health pool that you get from those. Plus obviously the MP cost and pop cap etc etc etc...
I also quickly checked for higher veterancy (meaning basically higher pen for the SU85), but still neglecting everything else:
Here the SUs are far ahead of the StuGs. Still, in about 79,7% of cases the StuGs fare at least "similar" or better. The biggest difference here is what you already mentioned: The SU85s basically cannot fail to stop the push (assuming here always that all shots hit and they also all hit frontally).
Another quick check on lower armor (260/270 like Panther/KV1) gave similar results, but obviously as expected: vet0 StuGs are even further ahead, vet3 SU85s are better but not as much as against Tiger/Pershing.