I think it really depends on how quickly you want to get to Melbourne. If you're in a hurry, flying is definitely the go as it's so quick, but I also like the slower, land-based modes of transport if I've got more time. Travelling by train is a great way to relax and enjoy views of the beautiful countryside while driving involves more effort but enables you to take any route you desire, stop when you feel like it and stay over anywhere that takes your fancy. Both the main coastal and inland routes from Sydney to Melbourne are very scenic, and there are so many beautiful spots to stop and explore on the way.
Definitely by plane, why spend 8 hours by train or car? But that really depends on whether you are stopping over to explore the small towns along the way and the main purpose of the journey was to go on a 'roadtrip'
It depends on how long you're planning to stay in Melbourne and if you'll need a car when you're there or not. If it's over a week, then driving and making it a road trip would be worth it. If under a week, flights are so cheap it might even cost as much as petrol!