SC is much less developed than gold coast but both are a bit bogan; US equivalent would be Pensacola for SC and Daytona Beach for GC; neither is Miami
Perth is very nice to live in, but Margaret River (3 hours drive south) is much nicer, and while small has much of the amenities of a big town. West coast advantage of sunset over the water and empty beaches with great surf; an order of magnitude less crowded than the east coast.
Adelaide has a surprisingly extreme climate (gets very hot and very cold) for a "Mediterranean" climate (which refers more to the rainfall pattern than temperature). Socially Adelaide used to be very conservative (due to migration pattern in 19th century) but that has changed in the past decade. Adelaide Hills (half hour drive outside of town) is a great small town, like Napa in California (it is all about the wine). Perth is a bigger town with more amenities, but ADL is 2 hours flight from MEL and civilization (or 3 hours from SYD).

The "nicest" part of the east coast is in NSW a bit south of the GC but much (much) more expensive, and that is Byron Bay and Coff's Harbour (halfway between SYD and BNE). If money is no object then that is by far the best choice, imho, based on your criteria. Second choice is GC, but you really have to see for yourself and spend some time in each location (several weeks each before you understand the ethos of a place). None are perfect and some of the flaws can be irritating long-term, though internet shopping has gone far in equalizing choices.