There is no question that Telstra has far superior coverage, but if you are on a budget many others are cheaper. Having a quick glance at the coverage maps for VIC (below), it looks like you will have to decide whether you want to use data away from the city or not.
Telstra
Optus
I don't thing the iphone will work on Optus Dual Band, but will work on NextG? Maybe someone else could confirm