depends on what you're into..
brisbane is based around doing things outdoors.. you have the gold & sunshine coasts not far away and yeh, its all outdoors stuff. swimming, surfing, lazy sunbaking on the beach, walking, stuff.
melbourne is better if you like little hidden away bars, gigs, clubs, movies/theatre/comedy shows, art gallerys etc. but the bonus with melbourne is you can go to the beach and swim until the sun sets at night or if you're up for a day/overnight trip you can head down to torquay/bells beach or the other side of the bay like port sea and phillip island for some awesome surfing/beaches. whoever named surfers paradise on the gold coast had obviously never been to some of the surf beaches in victoria.