I may be biased but Melbourne is my bet. There's always something on and it caters for all interests and age groups. My husband and I particularly like the theatre and we can always step out after a show for a meal or a coffee and are spoilt for choice. In Sydney we found it almost impossible to get a coffee after the theatre without actually having to buy a meal.
I live in Perth now however am from Sydney and we do not like Melbourians! There seems to be a rival thing, or rather there used to be.
I believe Sydney has everything including a great night life and especially if you know where to find it.
Of course it depends on what you want to do but in Sydney there is a great variety of night club, Clubs which have excellent entertainment, casinos, movie theatres, restaurants, cruises, theatre, just to name a few.