Hi there Wizard
I may be able to assist a little. I have lived all over Australia and am presently living in South East Queensland just north of Brisbane. South Australia was, for some period, regarded as a "rust bucket" economy but Adelaide (its capital) is regarded by many as the most civilised and genteel of Australia's major cities. However, the tendency is for its younger occupants to move interstate so it has an older population than, say, Sydney or Brisbane. Being at the Southern End of the continent it also has a more European style climate with clear delineation between the seasons (whereas Brisbane is sub-tropical (and far north Queensland is fully tropical)
But to address your specific queries:
1. Cost of living - House prices have escalated sharply in Queensland (especially Brisbane) over the last 5 years. Adelaide has also risen but woold be generally marginally cheaper for housing now than Brisbane. Queensland, however, has many sizable regional cities up its nearly 3000 kilometres of coast line (and quite a few inland as well) which South Australia does not have and many of these cities are both very liveable and have much cheaper housing than Brisbane or Adelaide (indeed some of these cities have very generous assistance programs to aid people who move there if they have special skills or trades that are in demand - eg, nurses, teachers, etc) . As for food prices, there is not a great deal of difference in terms of basics. Because of their different climates, though, some regional things tend to be more or less expensive. I would say, though, that South East Queensland was marginally cheaper for food.
2. Adelaide is quite compact with a population of around 1 million. Brisbane has almost twice that (and if you include the Gold Coast and Sunshine Coast regions immediately to the South and North of Brisbane the population of greater South East Queensland is closer to 3 million). South East Queensland is also growing rapidly (net migration to SE Qld from other states of Australia is roughly 32% of all migration within Australia making Qld the most popular destination for people moving interstate). South Australia's population is, by contrast, relatively flat (and ageing - it has the oldest median age of the country - almost 39 years whereas the median age in Qld is 36 years). Because of the explosive growth South East Queensland is experiencing growing pains and it is fair to say Adelaide's public transport would probably be better as a result (but then much more money is being spent now in SE Queensland with many large transport infrastructure project being constructed). SE Queensland has a good, modern electrified train system coupled with buses and ferries. I use public transport quite a lot and find it good here (especially given the price of fuel - which, btw, is the third cheapest on the planet in real terms after the US and Canada). Fuel is also subsidised in Qld by the government to the tune of 8 cents a litre whereas there is no subsidy in South Australia.
3. Income is a tough one because it depends on where you want to live precisely. Within 5 kms of the centre of Brisbane, for example, $450K or more would be about the starting price for an older 3 or 4 bedroom house. Where I am (Redcliffe - about 40 kms north of Brisbane), it is possible to get a good sized house like that for about $350,000. A little further north at Deception Bay or Caboolture, you could get a similar place for closer to $300K. With all the assistance given by the Government to those with kids a family of 4 would probably pay no effective tax until they hit about $60K pa in income. It would be possible to live on that with 4 kids (especially if you are able to purchase a house on arrival and do not have to rent). But I would think something closer to $80K pa would be the minimum to live reasonably comfortably with 4 kids. If both parents worked (maybe the wife part-time) you would probably be looking to achieve a combined income of $100K to $120K and that would make for a pretty comfortable existence for a family of 6 (taking into account the government assistance you would also receive - so called "middle class welfare").
4. Employment again depends on what your skills are (and of course migration is easier for those with specific skills) but as a general rule work will be much easier to find in Qld than South Australia. Unemployment in Qld is around 4% at present and even with a degree of pessimism over the world economy at present things are still going nicely and people are investing here. Both Qld and South Australia benefit from the resources boom but Qld is simply much larger (physically and financially) and has more stuff to sell (coal, iron ore, alumina, beef, tourism, etc) and so has done better from the China boom (China buys virtually everything that gets dug out of the ground in Qld). So far as food industry goes both Qld and SA have very mature markets for produce and both have extensive exporting bases. Queensland would have the edge in terms of restaurants largely because of the large numbers of tourists and tourist areas. South Australia has great food too and, arguably, the best wine area in the country. Adelaide would be more conservative (ie old fashioned) in some ways on the food front whereas Queensland is renowned for its "fusion" cuisine (ie unique blending of european (ie french/italian) cuisine with asian cusine (ie thai, malay, indian, chinese etc) which is a direct result of the migration here from Asian countries since the 70's. This is also as a result of more asian migration to Qld due to its closer proximity to Asia and similar climate. South East Qld would have a much more ethnically diverse population than Adelaide
For myself I've lived in many parts of Australia over the last 3 decades but I am very happy where I am now. At the moment I like the sunshine and laid back approach to things here in Redcliffe. It's close to a capiutal city but far enough away to avoid the hustle and bustle. Culturally, though, the Southern cities are more refined than SE Qld which is still seen by Sydney and Melbourne as something of a large country town (although that image is fast receding). My favourite Southern city is without a doubt Melbourne. I would happily live there but for the weather (there is an old saying in Australia that basically translates the phrase "Melbourne weather" as meaning experiencing all 4 seasons in the one day - and believe me it is no joke - you really do have wild swings in weather there on occasions. Just as a little plug for my patch here is the web page for our local Council which gives some facts and figures. I would look at some of the other coastal Qld towns as well (such as Bundaberg, Gladstone, Hervey Bay, Townsville and Cairns - all of which are keen to encourage migration and are all places I have spent extended periods in - and all are very welcoming and attractive)
http://www.moretonbay.qld.gov.au/discover.aspx?id=18239
Hope all that is of some assistance
cheers
Richard
