Sydney's alright (depending on where you are of course!). I'm not sure about Melbourne, but Sydney tends to get kinda scummy the further West you venture. Having said that, it's not the case with every suburb (I went to Paramatta today and the station is soo nice, possibly the nicest station I have seen in all of Sydney; and the mall is huge!! Not too shabby AT ALL!)
Eastern Surburbs or Lower North Shore are the most expensive (therefore, comparitively devoid of crims / junkies), and most of these suburbs have either water or harbour views, or have nice tree-lined streets, so they are really pretty. i.e: Vaucluse, Rose Bay, Kirribilli, Mosman, Double Bay, etc. Lot's of old, wealthy, society-types live in these parts.
The Inner West is really cool, trendy, and convenient as it's all pretty close to the City and pretty well serviced by trains. Inner West is where alot of the younger University population live, so it seems like a pretty hip and fun kind of place. i.e: Surry Hills, Glebe, Newtown, etc
Sydney is, as a city; expensive, shallow, vein, and superficial. It is the 'Los Angeles' to Melbourne's 'New York'. You can't compare the two, as they are so different. Personally, I love Sydney; however if I had the time and money to relocate down to Melbourne, I probably would.
NB: I vote that Gold Coast is better than both cities, hands down. It's like our Miami, or Las Vegas lol