I’ve flown both QF and UA between the US and Oz. The service on either is comparable to be honest. But like the other poster said, I’d go for the lower points total and use up your AA miles unless you have some other reason to hang onto them. One other note if you aren’t already aware: connecting international to domestic for your SYD-MEL flight is NOT a quick affair. You have to go to the domestic airport terminal, which is a couple of miles away and involves either a shuttle bus or taking the train. It will be fine if you have enough time - bare minimum is two hours, I recommend 2.5-3.