Unfortunately I am in New Zealand so can't comment on the specific options that are around in Australia but I will give a general answer that I think is universal.
I think the kind of antenatal care you decide on depends largely on the kind of birth you would like to have. Here we have the option of an OB, GP, or midwife in terms of the person/people who will be attending your birth. If there is no MEDICAL reason for it and you want an OB you have to pay for them. I knew right from the start that I wanted midwife based care as I had in mind the type of birth I wanted (home water birth)and I knew that a midwife would support me in MY choices.
I think with OB's you run the risk of all sorts of interventions you probably won't need or want as at the end of the day they are specialists in their field. I feel they should only be used in complicated and at risk pregnancies, not the normal everyday run of the mill ones like both of mine were. Think of it like wanting to get the oil changed in your car, you are not going to take it to a car dealership that specializes in selling cars when your neighborhood mechanic would do. Now if you had serious issues with your car, and a recall was made y the vehicle manufacturer, then you would take it to the dealership you bought it from as they would, more than likely, be the best person for the job.
So that is my 2 cents worth, not sure if it is helpful or not. So maybe the best place to start would be to decide what kind of birth you want and then research the best options to facilitate that and work from there. Of course if in the course of my pregnancies I had developed complications that needed the expertise of an OB, I would have had to examine my options again, but why start at the "top" if you don't need to iykwim.