IS3, IS4, IS7 How different can they be?

ARGO: Most people who went from the original IS tank seemed to like it. Looked good, good speed, and good firepower but something was missing......Armor. Most say they missed their KV3 because of this but then they get an IS3 and get thier first taste of a real good heavy tank. Pretty well rounded as heavies go, The IS3 really doesn't get much praise and most are dead set on going to the IS4 so I have some questions for serious debate.
Is there any reason to graduate up from IS3?
What are the pros and cons in doing so?
What noticable performance differences can one expect?
And most of all, once in an IS4 should one even bother to shoot for an IS7?