If you go to USA and get a quick PPL you will be able to convert it to Aussie PPL with air legislation and possibly other things to consider. You will be OK flying whatever you learnt in, but you will not be ready to fly some of the RAAus fleet of lightweight, fairly high performance, Rotax engined aircraft.
If you go the other way and learn with RAAus type aircraft, you will be pretty well equipped to step into the ageing GA fleet. You may not enjoy suddenly finding yourself in what feels like a heavy truck type of vehicle, but it will not be really dangerous.
The other difference between GA and RAAus is when it comes to cross country work. It is my belief that RAAus does not equip you for cross country flying. Just noting on this site that others have said they have several electronic means of navigation and they trust them to never break down.
I have experienced a GPS telling me to turn 45 deg when I could see my destination directly ahead, not once, but twice. High tech is great, but we need to know the basics.
Whichever way you go. Enjoy it.