I would personally go for the Whitlam dismissal because it has a lot more historiography, and it's not as recent as Keating.
Focus on the historiography of the dismissal rather than the actual history, this is what markers like to see as historiography is the main focus of the Extension History course.
To get you started, here is a newspaper article from April this year, that I found simply by googling "Whitlam dismissal historiography":
http://www.smh.com.au/federal-polit...e-real-word-about-whitlam-20130408-2hh5k.html
If worst comes to worst, just do something broad like "Evaluate the differing interpretations of the Whitlam dismissal".
You've chosen a really fascinating topic area, and I hope you do well! All the best