I think one reason why we are repeatedly seeing people say something like "I'd rather take <other faction's AT gun>" stems from faction design:
Ostheer does not have as many indirect fire options as Soviets. Their main source of indirect fire are mortars and then nothing at all until (if) they go T4. Everything they have is rather squishy (flames, explosions) and because of this and other reasons (no Oorah, more expensive mines) they have a harder time defending their AT gun. The things they'd use an AT gun for are mainly T70/T34 against which survivability is arguably more important than rate of fire. Against a Su-85 it just serves as a deterrent and can be taken out by the Soviet indirect fire solutions available in these situations (Su-76, Katyusha, 120 mm mortar).
Soviets already have potent indirect fire options or do not require them as much (survivable snipers, M3 flamers for taking/weakening positions). The squishyness of a Pak would not mean much to them because they could support it easier (mines, AT grenade, Oorah, merge, easier recrewing, doctrinal cloak). Rather, they'd take more potent non-Su-85 AT solutions because their amount and relative power of them is a bit lower (currently: combination of solutions less potent on their own).
And I think this is completely intentional from the design Devs.
Still, both lack from poor accuracy... their scatter stats should be improved so they are better against disabled and immobile (lack of enemy awareness) vehicles as well as against frontal charging vehicles.
Expanding on this, the ZiS crew is essentially a Conscript squad, they share the exact same health and armour. There's 6 men, they can be instantly reinforced from any nearby Cons squad, and are faced against Armour that's average at taking on Infantry, so Tank flanks are less threatening.
Now the PaK 40 crew, is
not like a Grenadier squad, as they lack the 1.5 Armour that Grenadiers have, making them more like Conscripts. There's also only 4 of them, they also can not be reinforced from infantry squads, and are facing Armour that is above average at killing Infantry, so even a tank from the front is a threat (in one extreme but rare example, a T-34 decrewed a fresh PaK in one hit + 2 Grenadiers bunched up next to it).
The threats they face are also different, a PaK is more far less sustainable under fire than a ZiS. A PaK can be molotov'd, grenaded, mortared, or even artillery'd by a ZiS! The weakness of the crew members and small size make it more difficult to keep alive on the front. A ZiS on the other hand, is threatened by rifle grenades and mortars, but has a larger squad that can be reinforced on the fly at the cost of another squad. This makes the ZiS overall far more sustainable even after multiple attacks.
Overall, unless the PaK is recrewed by Grenadiers*, I don't think the 30% faster RoF makes up for all of its' shortcomings. I personally think the ZiS is the better gun in this case.
*
Only Grenadiers. Panzergrenadiers are the same as Grenadiers except for their weapons and abilities, which are lost when you crew a weapon. This would mean you pay more to reinforce the PG's AND PaK, but gain no benefit for using PG's over Grens. Osttruppen are much weaker than the original crewmembers, and would not last half as long. Pioneers would be totally identical to the crew they replaced.