Hornsby's a great leafy suburb (worked there a lot!), but I think its a bit far away for anyone coming interstate, from a plane, for example.
One of the things I liked about the Brisbane con was that it was right in the heart of Brisbane CBD. I would like to have gone and investigated Brisbane CBD a bit more, considering the small time I had up there. But the idea was right. By putting the con in the centre of the city, you attract more people who might make an extended weekend of it, and put all the facilities close to the con, making it even more attractive, and easy to get to.
Sydney's a great city, full of cool attractions, and getting people to come to the con, not just for the con, but for the city too, is a great added incentive.
My 2 cents!