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).
Well considering my whole point was that differing circumstances can heavily influence that match-up, more evidence to show that it's not as simple as 'this beats that' is perfectly welcome.
The StuG never gets that self spot or range, just as the SU-85's vet is different with penetration having implications for units outside a matchup with a StuG. And while the SU-85 slows down on self spot, I don't think anyone can deny the defensive advantage.
They're different units with different costs and function. This is not the same as comparing the differing costs and function of the Panzer 4 or Panthers from Ostheer and OKW.