I would get a desktop if you don't need portability for it. Most of the time you'll get more power. Also, if you don't plan on have hundreds of applications open at a time get less ram and a better video card.
As an Australian, the options are pretty limited sadly but with a $900 budget, if you're still in school and don't have a laptop, I'd suggest going with that. If you're looking for something better like a desktop but is also very portable, I'd look into an Alienware alpha. As an owner, they are much better than their reviews. -Cam