I've lived in both, and I can assure you Sydney's startup/dev scene is far larger. For one thing, basically all the multinational IT giants (Google, Facebook, Amazon etc) have their Oz HQs in Sydney.
Losing Startcon to Sydney was one of my top reasons other than that international startups (particularly fintech) seem to gravitate towards Sydney.
Melbourne does have some noticeable startups - Carsales.com.au, realestate.com.au, envato, seek but I think there isn't enough done to phrase them. If you are in Melbourne however I'd recommend coming a Inspire9 pitch night or a SiliconBeach meetup. Finally Andrew Hyde's writing speaks of some of the strengths in Melbourne http://andrewhy.de/why-your-startup-should-visit-melbourne/