Sergei is and was a better boxer than JDS, there is really no argument about it.
What convinces you that JDS has a granite chin? Who has really tested his chin? When has he been solidly cracked in the chin and by whom?
I have no real ill will against JDS, but he's turning into the flavor of the month.
The only thing he really does well is throw his hands, particularly his right.
Overeem's left hook will probably land first if they throw at the same time.
Overeem has better stand up overall, and he also has good kicks and knees. Overeem has also shown better submission skills in his career (though this is yet another area in which JDS is untested). And, JDS is not taking Overeem down.
Overeem will destroy JDS. I'm thinking a first round knockout is a likely scenario.