have lived in Adelaide and gold coast and in the end it depends!
the sunshine is not year round anywhere in Oz Adelaide gets a decent long summer (3 months or more) and very mild winters and very very little rain with very low humidity year round, some very nice beaches. def has some of the best schools and university in Oz. The rental and housing prices are much cheaper in Adelaide
where as gold coast is 20 degrees ish year round but it rains a lot and it is very humid most of the year.
I found the gold coast a bit like a 'spanish holiday' resort cheesy and commercial (not a plus for me but it may be for some)
in my opinion Adelaide would be more cosmopolitan than the gold coast, better restaurants and a classer night out.