We have been chatting about this in the office and I thought I would put some input…
While it may be cheaper to host the conference in Brisbane, IMHO there is a lot more to do in Sydney… especially touristy stuff….
The Australia Zoo is approximately 45-60 minute drive from Brisbane…
Taronga Zoo is about 15-20 minutes on a ferry from darling harbour and includes a beautiful view over sydney.(http://www.simonho.org/images/Australia/Sydney_Harbour.jpg).
Most sights to see are within 20 minutes of the city if you even have to leave the city…
Also, Brisbane would be a secondary flight for most people flying over as most international flights come into Sydney and Melbourne, not to mention that the tourist attractions are so far apart that you’d need to hop on another plane to see them from Brisbane…
The nightlife in Sydney is endless… and you’re always bound to see a famous Aussie! Mostly footballers, but theres nothing wrong with that if you’re a lady 
We have Kings Cross and Newtown – the largest population of Queens in the world after Miami 
Plus we have the Harbour Bridge and Opera House – you can’t go past a bridge climb!
http://www.sydney.com/ – this website has a lot of info…
Anyway just my 2 cents…