DHL is gonna cost as much as express international or global express guaranteed, and will be similar sort of service. So sticking with Auspost cos it is closer to you, if you are going to pay more for the higher level of service, would be what I would do.
One thing I have noticed with DHL is that they will do all the customs clearance on the flight from US to Au that or does save a little time for the quicker service.
As for UPS I have found if they pay for a 7 day service for example, even when it is here in Australia in the warehouse at day 5, they will not bother shipping it out until day 7.
Ersk