Originally Posted by
bseller
That would seem to me to be the better option, since 4 nights in SYD would (or COULD) be packed with things to do - no need for car.
While I realise the OP is on a honeymoon so a fair amount of the activities can be "created", 12 nights in Tahiti seems like a long time.
I'd counsel the OP still consider a combo Melbourne/Sydney option, if they don't want to rent a car there is plenty to do in those cities to fill in the time.