If you have limited time, as you do, I'd do Sydney and skip Melbourne to allow more time elsewhere. Yes, to an Ozzie it is quite a different culture but other than the afternoon trip to see the penguins, there's not much there that you won't find in any other city. And I'd definitely put the Twelve Apostles into the same category as Uluru/Olgas -- just about half as good as claimed (at best). If you've driven up SR1 in California, or plan to, you can definitely skip those Apostles.