We moved to Adelaide from Melbourne last August, for us Adelaide was always our first choice but we know the IT job market was very limited so thought we would give Melbourne a try first. There was huge job choice there, the public transport is very good and is much larger and more urban than Adelaide. Traffic is much worse there and takes a long time to get around by car. For me I never really gelled with Melbourne, being from a small village in the UK, I found it too busy and built up. It has loads more going on, but that wasn't important to us. It depends what lifestyle you want and what you are into. In Adelaide we can afford to live by the beach, with only a short drive to the hills, wineries, and the city. It does feel tiny but we like it that way, the only worry is the job market and we would never have risked coming to Adelaide without a job offer first. Feel free to ask my any questions about either city.
Good of luck with your choice, make the right decision, moving interstate is almost as expensive as moving from the UK!
Julia