You haven't been to either city have you? Sydney's night life is trash, there's next to nothing going on after 9PM. The city is super expensive and confusing to navigate.
There's a ton of tourist attractions in Melbourne:
The Star Wheel, the MCG and all the other stadiums, Flinders Street Station, The Aquarium, Art Museum, Crown Casino (some people count it) and the Melbourne Grand Prix. There's probably more I'm forgetting to mention, but imo Sydney is very overrated