IS-2 isint really more durable because of stronger ostheer AT guns. The time it takes for a zis-3 to kill a tiger is exactly the same as a pak-40 takes time to kill an is-2.
The pak40 has a harder time penetrating a Is2 than zis penetrating a tiger.
The IS2 is still faster, means you can gtfo quicker when a situation becomes hairy.
I usually manage to keep my Is2 alive longer than my tiger, hence the reason why i completely stoped using tigers since they are just vet food for tank destroyers. Especially after the Jackson pen buff.