I studied abroad in Sydney and it was the best 6 months of my life. I highly reccomend it. Nothing against London, but I think you would have much more fun in Australia. Also, there is so much to see in Australia and New Zealand that you might never get to see if you don't study abroad there. If you like beaches and natural beauty, Australia is definitely the way to go.
And as someone else mentioned, London is a lot more expensive. Sydney is not cheap, especially since the US dollar went into the toilet, but it's still much better than London. As a very rough rule of thumb, stuff in the US, Sydney and London cost the same nominally, but 1 US$ = 1.27 AU$ = .52 GBP. London is rated as the 4th most expensive city in the world and Sydney is 19th. (
http://www.finfacts.ie/costofliving.htm )