Not entirely true. SU-85s have range and can self spot. That can just as easily translate to an SU-85 killing a StuG and taking 0 damage.
If it can self spot, it can't outpace a Stug. Even if both units have sight on each other, the Stug is still faster. You can win if you have LOS AND you back up prior to the Stug starts moving up. Overall it's a match up decided by RNG mostly (whoever miss probably losses).
Penetration means shit, and hits to kill are equal although the Stug has less health. On the same time it takes for the Su85 to do 4 reloads, a Stug makes 5. I'll gladly trade a Stug for an Su85. Most people are scared to do that kind of push.
Any kind of veterancy puts the edge on the Stug (TWP, vet2 no longer offers RoF for Su85 and it does on the Stug, vet3 RoF vet is higher on the Stug).