.com.
Unless you are specifically and only marketing in the geoloc.
Looking again at your actual question though...
The one is a subfolder (.com/au).
The other is a domain level URL (.au)
DId you mean that? Are you asking whether a subfolder on someone else's site is better than your own site? If so though, the answer for many reasons is NO.
I don't often say something is so with no nuance or context, but this is an exception. Unless the .com is Google itself or Wiki then get your own site. And even if it IS Google or Wiki, still probably get your own site