Yes, armor is armor, but effectiveness of this armor is dependent purely on the penetration of AT it faces.
Unless you want to argue that 300 armor against 200 penetration is the same as 300 armor against 140 penetration or (PaK40 vs USF AT gun).
And anti tank guns are not even close to being the same. Penetrations are different, rates of fire are different(both in favor of axis).
Sure, not everything is a Panther, but the most common AT, AT guns and zookas/shrecks have wide enough gap in performance to say 300 allied armor is nowhere near being equal 300 axis armor.
Its not theorycrafting, its not myths or fairy tales, its hard facts based on in game stats of units in question, you can't really argue that, unless you want to argue stats.
Oh, and I'm not saying ISU armor is weak, its fine.
I'm saying its not equal to Tigers in effectiveness due to meaningful and noticeable AT difference.
When I spoke of AT guns I thought mostly of ZiS-3 vs Pak40 I admit, which are similar in penetration. But even the American gun has an ability that increases its penetration also to a similar level, and since the ability lasts 30 seconds or so I think it makes very well sense to compare it to them.
Rate of fire may be different, but you know, there are so many things that can affect tank combat a lot outside of that. The ZiS-3 crews might survive longer than Pak40 crews because they have more health, Guards Rifles can button tanks, which effectively gives you enough time for some extra shots while he is standing there doing nothing, and Mark Vehicle commander ability reduces the amount of shots you require.
Now you can rightfully say that the OKW Command Panther has a similar ability to Mark Vehicle, even if that is more rare. You could also say that not every Russian commander has access to those things, I could say now that the ISU commander has coincidentally access to both Guards Rifles and Mark Vehicle and then also to the Sturmovik Triple Bomb drop that if it hits will do massive damage to vehicles.
We were not just talking about that commander, I am aware, but all I want to say is there are so many different things that could affect the outcome of the fight that I find the general statement that Axis armor is significantly more valuable than Russian / Allied armor not very accurate or useful.