Seems both Tiger and ISU have 200 pen far.
The Tiger has 25% more DPM. TTK against a medium tank is roughly 50% longer for the ISU. The Tiger has much higher effective accuracy. The ISU can not chase effectively, needs to be stationary to fire and is a casemate with only -15/15 max traverse and very slow gun traverse (12 d/s). It's also very slow and needs to manually switch shells. The only thing it has going for it is range and having Mark Vehicle in the same commander which can in some cases decrease its TTK.
So yes, even though the ISU is decent at fighting tanks (although 200 pen is barely enough to reliably penetrate a Panther), it is not even close in terms of AT performance to the Tiger.