This seems like a very touchy site at the moment, so at the risk of upsetting several othewise-mild members, I thought I'd add my view on the OP's question: if I were a 17 or 18yo, I'd want to go to Brisbane over Adelaide every time. If I were a parent of the teenager, I'd vote for Adelaide.
Both places have their attractions and their downsides, but as a teenager I'd be looking for the most lively (but I recognise not all teenagers are the same - I can only go on what I looked for in life at that age) and Brisbane wins on that basis.
Regarding moving interstate, the reality is most teenagers don't move away unless with their parents; if people move independently, they are most likely to do this once they hit 25yo. In percentage terms, more younger people (25-29) move interstate from Adelaide than from any other mainland capital, and SA has the slowest growing population and is ageing the fastest, so it is struggling to keep its share of younger people and doesn't manage to attract them in enough quantities from other states. In recent years, though, the number of younger people leaving SA has decreased slightly. There are many factors at play with all this, so make of it what you will.